Vindicar

Okay, something is definitely off.
I tested it by hollowing a slime chunk in the middle of an ocean, standing 40 blocks away (out of spawn block zone) and then repeatedly killing other mobs via commands. No slimes spawned.
The world has been generated in 1.12, but it was MCEdit'ed later. I dunno if it's relevant.

I seem to be having trouble with slime spawns.
I'm using MineAtlas to find slime chunks, and while it worked for me before, currently the chunk I'm using (chunk -10,15, X~=150 Z~=250).
I cleared that chunk down to level 10 and did try my best to light up the caves around, but still nothing spawned there.
Still, it's possible something else is interefering with the spawns, like mobs in other caves...

Got the same thing happen in 1.12.1.
After placing a map into item frame and breaking it several times the green stationary marker stopped appearing. The white player marker is still working.

Another observation: it seems that slimes spawn in the slime chunk if I go quite a bit away from the chunk.
Since they also instantly despawn when there are no players in 128 blocks range, it leaves only a very small window open for them to spawn.

I had 3 separate instances of them spawning in the farm and leaving drops, every time after I travelled away and returned.
Note that normally I don't go into 24 blocks range that'd prevent slimes from spawning.

I'll drop a comment here, since I accidentally posted a dup of this issue: this definitely affects 1.9 prerelease up to and including pre4.

Same bug affects me, but I am able to fix it by force-reloading resource pack (F3+t).

Server specs:
CPU: 4x Intel(R) Atom(TM) CPU D525 @ 1.80GHz
RAM: 4GB

System usage:
Cpu(s): 2.8%us, 0.1%sy, 0.0%ni, 97.0%id, 0.1%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 4032564k total, 3694464k used, 338100k free, 132536k buffers
Swap: 4173820k total, 35744k used, 4138076k free, 2130056k cached

Java version "1.7.0_51"
OpenJDK Runtime Environment (IcedTea 2.4.4) (7u51-2.4.4-0ubuntu0.12.04.2)
OpenJDK 64-Bit Server VM (build 24.45-b08, mixed mode)

Game rules:
gamerule doMobSpawning
[Server thread/INFO]: doMobSpawning = on

--------------------- server.properties ---------------------
#Minecraft server properties
#Sat Jan 25 20:27:36 MSK 2014
generator-settings=
op-permission-level=4
allow-nether=true
level-name=world
enable-query=false
allow-flight=false
announce-player-achievements=true
server-port=25565
level-type=DEFAULT
enable-rcon=false
force-gamemode=false
level-seed=4586832350340175357
server-ip=
max-build-height=256
spawn-npcs=true
white-list=true
spawn-animals=true
texture-pack=resource-pack.zip
snooper-enabled=true
hardcore=false
online-mode=true
resource-pack=
pvp=true
difficulty=1
enable-command-block=false
server-name=Vindicar's Server
player-idle-timeout=0
gamemode=0
max-players=8
spawn-monsters=true
generate-structures=true
view-distance=10
spawn-protection=16
motd=Welcome to Vindicar's server
--------------------- server.properties EOF ---------------------

I disagree. Currently I have server render distance set to 10, and client render distance maxed out.
Still no hostile mobs spawning AT ALL.

I've encountered the bug too after upgrading to 1.7.4, only in my case mobs have ceased to spawn completely. Mob spawner blocks are still functioning properly, but that's it. Restarting the server doesn't help, neither does switching to Peaceful and back. Log doesn't show any errors, aside from rare "Can't keep up" warning.
If I start new local world in 1.7.4 client, then I get small amount of monsters, but I can't get monsters to spawn on my server at all.

Server info:
$ java -version
java version "1.7.0_25"
OpenJDK Runtime Environment (IcedTea 2.3.10) (7u25-2.3.10-1ubuntu0.12.04.2)
OpenJDK 64-Bit Server VM (build 23.7-b01, mixed mode)
$ uname -a
Linux VindicarServer 3.2.0-57-generic #87-Ubuntu SMP Tue Nov 12 21:35:10 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux

I confirm that the bug is still present in 1.7.2

I have two clones of the same map in frames, third in my inventory (and two more in chests).
Whenever I place a third clone on a wall, third green marker fails to appear on the map I'm holding. If I drop my copy and pick it up, then all green markers are present but white marker is gone. If I remove map from the wall, white marker appears back and third green marker disappears as it should.
It only happens to the map I'm holding, maps on the walls function normally.

For me, snapshot 13w36b seems to mitigate CPU issues somewhat. But it might need some more work.

@Eris. Unteresting. But my statement above is based on my observations. Maybe it's a heisenbug.

I must add that yes, it affects only villagers. I found a zombie spawner away from the village, and it works just fine.

So, Mojang stays silent about what seems to be a critical bug, having solution code written for them already.
Could any of them at least comment here so we know they are following the issue?

Still not fixed in 1.6.2 release.

Bug appears 10 times out of 10 for me.
Could you specify name&version of JRE package you use?
Maybe I'm using wrong JRE.