How to reproduce
by @unknown
Double click on the grey arrow on the world selection screen
This will cause the game to load the world twice (single click on the arrow and double click) (discovered while testing MC-92079).
IMPORTANT : I only noticed this bug once and could not reproduce it.
When loading a save, chunks started to display normally but I slowly sank into them (solid blocks included) when I stopped flying. It was exactly like if I was in an unloaded part of the world but everything seemed to be loaded and was displaying correctly (I do not remember if entities including mobs were, though).
Confirmed by someone on Twitter : https://twitter.com/AkterSnurra95/status/634805930052481024
The sinking stops at bedrock level.
Game Output (15w47a)
[23:01:10] [Server thread/INFO]: Generating keypair
[23:01:10] [Server thread/INFO]: Preparing start region for level 0
[23:01:10] [Server thread/INFO]: Changing view distance to 12, from 10
[23:01:11] [Server thread/INFO]: Marcono1234[local:E:119f6c66] logged in with entity id 214 at (0.04227024881031996, 65.6183020148442, -44.376317253900716)
[23:01:11] [Server thread/INFO]: Marcono1234 joined the game
[23:01:11] [Server thread/INFO]: Starting integrated minecraft server version 15w47a
[23:01:11] [Server thread/INFO]: Generating keypair
[23:01:12] [Server thread/INFO]: Preparing start region for level 0
[23:01:12] [Server thread/INFO]: Stopping server
[23:01:12] [Server thread/INFO]: Saving players
[23:01:12] [Server thread/INFO]: Marcono1234 lost connection: TextComponent{text='Server closed', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null, insertion=null}}
[23:01:12] [Server thread/INFO]: Marcono1234 left the game
[23:01:12] [Server thread/INFO]: Stopping singleplayer server as player logged out
[23:01:12] [Server thread/INFO]: Saving worlds
[23:01:12] [Server thread/INFO]: Saving chunks for level 'Test World New'/Overworld
[23:01:12] [Server thread/WARN]: The save is being accessed from another location, aborting
[23:01:12] [Server thread/INFO]: Saving chunks for level 'Test World New'/Nether
[23:01:12] [Server thread/INFO]: Saving chunks for level 'Test World New'/The End
[23:01:12] [Server thread/INFO]: Changing view distance to 12, from 10
[23:01:12] [Server thread/INFO]: Marcono1234[local:E:3b27436f] logged in with entity id 243 at (0.04227024881031996, 65.6183020148442, -44.376317253900716)
[23:01:12] [Server thread/INFO]: Marcono1234 joined the game
[23:01:25] [Server thread/INFO]: Saving and pausing game...
[23:01:25] [Server thread/INFO]: Saving chunks for level 'Test World New'/Overworld
[23:01:25] [Server thread/INFO]: Saving chunks for level 'Test World New'/Nether
[23:01:25] [Server thread/INFO]: Saving chunks for level 'Test World New'/The End
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Does MC-86937 describe your issue?

No. The chunks appeared to be normal. There was no apparent glitch. I was only slowly sinking in the blocks, through the ground, just like when you’re in an unloaded part.

same thing happend to me like 5 times already!
It looks like everything is loaded normaly, no invisible blocks or holes, but you sink through the displayed blocks

Can you reproduce the issue?

This happened to me as well in the w33 snapshots, as I recall.

Just got this to happen again in 15w34d. It is impossible to interact with the world; for instance, /setblock ~ ~ ~ air
does nothing.

In my case I was not even in a hole : blocks below me as well as the whole chunk were displayed.

That picture is simply there to illustrate that the player can clip through the world. I was not in a hole, but rather inside sandstone.

Ah ok, I see the rails above you now, my bad.

Got it again, and it always seems to happen with the following line in the log before leaving the world the previous time:
[Server thread/INFO]: anon_ymus lost connection: TextComponent{text='Disconnected', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null, insertion=null}}

Confirmed for
15w35b
Forced crash
---- Minecraft Crash Report ----
// I bet Cylons wouldn't have this problem.
Time: 24.08.15 20:41
Description: Manually triggered debug crash
java.lang.Throwable
at azh.s(SourceFile:1622)
at azh.av(SourceFile:926)
at azh.a(SourceFile:375)
at net.minecraft.client.main.Main.main(SourceFile:125)
A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------
-- Head --
Stacktrace:
at bhn.a(SourceFile:361)
-- Affected level --
Details:
Level name: MpServer
All players: 1 total; [bjm['Marcono1234'/2254677, l='MpServer', x=-468,25, y=61,33, z=-515,58]]
Chunk stats: MultiplayerChunkCache: 624, 624
Level seed: 0
Level generator: ID 01 - flat, ver 0. Features enabled: false
Level generator options:
Level spawn location: -469,00,4,00,-501,00 - World: (-469,4,-501), Chunk: (at 11,0,11 in -30,-32; contains blocks -480,0,-512 to -465,255,-497), Region: (-1,-1; contains chunks -32,-32 to -1,-1, blocks -512,0,-512 to -1,255,-1)
Level time: 719936 game time, 1449 day time
Level dimension: 0
Level storage version: 0x00000 - Unknown?
Level weather: Rain time: 0 (now: false), thunder time: 0 (now: false)
Level game mode: Game mode: creative (ID 1). Hardcore: false. Cheats: false
Forced entities: 2 total; [bjm['Marcono1234'/2254677, l='MpServer', x=-468,25, y=61,33, z=-515,58], wr['Armor Stand'/2254676, l='MpServer', x=-468,50, y=57,00, z=-520,50]]
Retry entities: 0 total; []
Server brand: vanilla
Server type: Integrated singleplayer server
Stacktrace:
at bhn.a(SourceFile:361)
at azh.b(SourceFile:2361)
at azh.a(SourceFile:384)
at net.minecraft.client.main.Main.main(SourceFile:125)
-- System Details --
Details:
Minecraft Version: 15w35b
Operating System: Windows 7 (amd64) version 6.1
Java Version: 1.8.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 230734312 bytes (220 MB) / 371843072 bytes (354 MB) up to 1060372480 bytes (1011 MB)
JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
Launched Version: 15w35b
LWJGL: 2.9.4
OpenGL: AMD Radeon HD 6800 Series GL version 4.5.13399 Compatibility Profile Context 15.200.1062.1004, ATI Technologies Inc.
GL Caps: Using GL 1.3 multitexturing.
Using GL 1.3 texture combiners.
Using framebuffer objects because OpenGL 3.0 is supported and separate blending is supported.
Shaders are available because OpenGL 2.1 is supported.
VBOs are available because OpenGL 1.5 is supported.
Using VBOs: Yes
Is Modded: Probably not. Jar signature remains and client brand is untouched.
Type: Client (map_client.txt)
Resource Packs:
Current Language: English (US)
Profiler Position: N/A (disabled)
CPU: 6x AMD Phenom(tm) II X6 1055T Processor
Note that Level generator options:
is empty. In previous versions it showed the preset for the world (in this case redstone ready), however this seems to be not caused by this bug.

This seems to happen often when quiting and reopening a world very fast.

Just had a case of this, when recreating a bug. It was like Gamemodes Spectator, Creative, and Adventure had a baby. It was Gamemode Glitch
So, it is confimed for 15w36c. When it happens, you have to reload the world to have it stop.
Confirmed for 15w37a
There's a certain probability collapse at world loading screen.
Reproducing video :
https://www.youtube.com/watch?v=Z8ApB0D9xPE
Confirmed for 15w38a
Confirmed for 15w38b
Confirmed for 15w39b
Confirmed for 15w39c

The only way to reproduce this when the player reloads the game too fast.
Confirmed for 15w40a
Confirmed for 15w40b
Confirmed for 15w41b
Confirmed for 15w42a
Confirmed for 15w43a
Confirmed for 15w44a
Confirmed for 15w44b

Yes, down to bedrock to stop.
It occurs in all modes.

Edit: just looked at previous Comments,
Confirmed for 15w45a
Confirmed for 15w46a

Confirmed for
15w47a I also had this rather strange
Game Output
:Game Output (15w47a)
[23:01:10] [Server thread/INFO]: Generating keypair [23:01:10] [Server thread/INFO]: Preparing start region for level 0 [23:01:10] [Server thread/INFO]: Changing view distance to 12, from 10 [23:01:11] [Server thread/INFO]: Marcono1234[local:E:119f6c66] logged in with entity id 214 at (0.04227024881031996, 65.6183020148442, -44.376317253900716) [23:01:11] [Server thread/INFO]: Marcono1234 joined the game [23:01:11] [Server thread/INFO]: Starting integrated minecraft server version 15w47a [23:01:11] [Server thread/INFO]: Generating keypair [23:01:12] [Server thread/INFO]: Preparing start region for level 0 [23:01:12] [Server thread/INFO]: Stopping server [23:01:12] [Server thread/INFO]: Saving players [23:01:12] [Server thread/INFO]: Marcono1234 lost connection: TextComponent{text='Server closed', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null, insertion=null}} [23:01:12] [Server thread/INFO]: Marcono1234 left the game [23:01:12] [Server thread/INFO]: Stopping singleplayer server as player logged out [23:01:12] [Server thread/INFO]: Saving worlds [23:01:12] [Server thread/INFO]: Saving chunks for level 'Test World New'/Overworld [23:01:12] [Server thread/WARN]: The save is being accessed from another location, aborting [23:01:12] [Server thread/INFO]: Saving chunks for level 'Test World New'/Nether [23:01:12] [Server thread/INFO]: Saving chunks for level 'Test World New'/The End [23:01:12] [Server thread/INFO]: Changing view distance to 12, from 10 [23:01:12] [Server thread/INFO]: Marcono1234[local:E:3b27436f] logged in with entity id 243 at (0.04227024881031996, 65.6183020148442, -44.376317253900716) [23:01:12] [Server thread/INFO]: Marcono1234 joined the game [23:01:25] [Server thread/INFO]: Saving and pausing game... [23:01:25] [Server thread/INFO]: Saving chunks for level 'Test World New'/Overworld [23:01:25] [Server thread/INFO]: Saving chunks for level 'Test World New'/Nether [23:01:25] [Server thread/INFO]: Saving chunks for level 'Test World New'/The End
I did not had seperate Minecraft instances running and the server also was not stopped completely, for example the EnderDragon still moved properly but I could only interact once with a boat and after that I was unable to do anything else


Confirmed 15w49b

I've reproduced that in multiplayer on 1.8 (on a spigot server). I think I have a recording of it somewhere. It loaded the world, but was unloading and reloading stuff, and generally behaving weird. (I believe it does have the same cause, though - double-clicking the server? Or something like that?)
I also reproduced that on 15w49b just now (did record that).
Confirmed for 15w50a
Confirmed for 15w51b
Confirmed for 16w02a

Hitboxes aren't showing up like the blocks are not solid.

It's because the chunks are "loading" because you sink slowly like on loading chunk
Confirmed for 16w05a

confirmed and replicated in 16w06a