Colton Jelsema

Confirmed 19w11b

Crash after opening a barrel placed before updating to the snapshot.

Unable to open world after the crash, as it crashes again with the same error.

later noted this is related to MC-145868

Happened to me in 18w16a.
Initially shows:

17:13:49	bjs	Failed to save chunk
java.lang.NullPointerException
	at gz.a(SourceFile:91)
	at bjs.a(SourceFile:426)
	at bjs.a(SourceFile:190)
	at sg.b(SourceFile:225)
	at sg.d(SourceFile:287)
	at sh.k_(SourceFile:199)
	at net.minecraft.server.MinecraftServer.w(SourceFile:696)
	at net.minecraft.server.MinecraftServer.v(SourceFile:629)
	at dbc.v(SourceFile:156)
	at net.minecraft.server.MinecraftServer.run(SourceFile:534)
	at java.lang.Thread.run(Thread.java:745)
17:13:56	bjs	Failed to save chunk
java.lang.NullPointerException
	at gz.a(SourceFile:91)
	at bjs.a(SourceFile:426)
	at bjs.a(SourceFile:190)
	at sg.b(SourceFile:225)
	at sg.a(SourceFile:251)
	at sh.a(SourceFile:716)
	at net.minecraft.server.MinecraftServer.a(SourceFile:448)
	at net.minecraft.server.MinecraftServer.v(SourceFile:647)
	at dbc.v(SourceFile:156)
	at net.minecraft.server.MinecraftServer.run(SourceFile:534)
	at java.lang.Thread.run(Thread.java:745)

then begins showing

Failed to save chunk
java.lang.NullPointerException

any time the game would try to save chunks, specifically when I went to the nether and back.

Can be seen when using an item that boosts max health and then relogging onto world. When joining the world, your health will be reset to the base max health of 20 before your max health is increased.
Example item:

give @p iron_boots 1 0 {AttributeModifiers:[{Slot:feet,Operation:0,UUIDLeast:1,UUIDMost:1,AttributeName:generic.maxHealth,Amount:10,Name:CG}]}

also, confirmed for 16w41a

confirmed for 16w41a

confirmed for 16w41a

confirmed for sprint flying in creative and high spectator speeds for 16w40a. especially prevelent with higher ping

Having a health value above your maxHealth attribute and then taking certain types of damage cause the false death.
fire, poison, suffocation, and drowning cause false death.
melee, fall, momentum damage do not cause false death

confirmed for 1.9 pre-2

Confirmed for 1.9-pre2

Confirmed for 1.9-pre2

Although happy about the fix, I was expecting some kind of server property that goes along the lines of a maximum momentum at which the anti-cheat kicks in.
Seems like it would fix all parts of the bug, and fit with it only affecting servers.

Tan Chn Cheern 's idea worked for me. Thank you!