In my creative flat world, on peaceful, I was flying and looking around until on the side of my screen, I saw a slime for a second. But it disappeared right away. I think this is a bug.
What I expected to happen was for there to be all hostile mobs to be turned off in peaceful.
Steps to Reproduce:
1. Fly, in creative, in a flat land world.
2. Set your difficulty on Easy.
3. Fly around again, without hurting mobs.
4. Set your difficulty on peaceful.
5. Fly around, and you will see, for a second, slimes.
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Cannot confirm. Never saw any mob appearing for a short time in peaceful mode.
Confirmed. The scoreboard in the screenshot increments 1 every tick a slime is in the world.
The command:
execute @e[type=Slime] ~ ~ ~ scoreboard players add Spawned Slimes 1
Reproduced on singleplayer superflat (could not in default generation). And yes, the slimes are visible for around half a second.
Also, this is only for naturally spawning slimes. Slimes spawned from a mob egg will not trigger the scoreboard.
I can confirm this using /testfor and scoreboard. My commands:
/testfor @e[type=Slime]
/scoreboard players add count Slimes 1
This although happened to me using the newest snapshot with the slim blocks and barrier blocks. I don't lag either i usually have 100-200 maybe 300 FPS because i got a new Lenovo laptop.I was flying around and I see a slime for a 1/2 second. Although I don't think it happens in regular updates meaning if I don't use any of the snapshots just a plain update Ex; 1.7.5. This happened when i was testing out the snapshot to see what was new. And by the way this happened on Super Flat world so try that instead... If you still do not see it then I will record some proof!
The proof is on the way! I am walking around on my new map on super flat. Then a few slimes just spawn and despawn for a 1/2 second! Hope this convinces you enough that there is a bug on the 14w11b snapshot! The video of the PROOF is above where the attachments are! If you don't know the name of the video. The name is "mob glitch.wlmp".
Appears to be fixed as of 14w18b. Created a superflat world and no slimes have spawned.

According to MC-54429 : Not
Confirmed for 14w18b.
Confirmed for 14w19a.
Confirmed for 14w29b

14w34c Just started happening for me in Super Flat.
Possibly lag on your system, from what I understand the current design does spawn slimes in peaceful mode and then "immediately" kills them. Even a slight, temporary lag can make that "immediate" happen for just a split second. Just best guess there.