When in Peaceful mode, slimes will quickly spawn and despawn naturally. Command blocks also find these slimes with the /testfor @e and /kill @e[type=Slime]
This was tested in Superflat mode.
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Its not using spawn eggs. The mob spawns naturally and despawns right after. It used to happen with all mobs in peacefull on minecraft 1.5.2 servers
Cannot confirm
I'm pretty sure this isn't a bug, as all peaceful does is to program hostile mobs to immediately despawn, rather than preventing them to spawn.