Mobs sometimes glitch when walking across a floor made of half slabs without anything under it. I've directly seen two results, and believe there is a third one:

• More commonly, a "ghost" of the mob drops through the half slab floor; it can't be attacked or interacted with in any way. It sometimes makes yo-yo movements and after a time jumps to where he fell from. Appears to happen when the mob glitches only on the client.

• Less commonly, the mob really drops through the floor.

• I believe sometimes the glitch happens only server-side; I've been hounded by invisible skeletons under my mob grinder completely made of half-slabs. Not sure, though, because I obviously can't see the invisible skeletons falling through the floor.

Saw all the above problems in Survival, in a heavily modded game, when I made a mob grinder almost entirely out of half slabs in a Skyblock-like map in order to save materials; reproduced the ghost mobs issue on Creative, in both Vanilla 1.4.2 and the 1.4.3 pre-release. The other issues are way rarer.

The attached file is a creative world with a copy of my mob grinder entirely made of half slabs; I typically see a few ghost mobs in less than a minute after loading that world.