I have a zombie grinder where the mob lands on a half slab. Then there is a full block gap where other half slabs are attached to the bottom of the "tube" where they fall down (see pictures). Whenever a new zombie falls and hits the half slab, most of the other zombies get pushed outside and go all over the room. You can even hit the mobs when they appear to be outside of the holding area, so it's not just a graphical bug. After a second or so, the mobs get pulled back into the holding area where they should be. Note: I have not tested this in other scenarios (i.e. mobs falling on full blocks, filling in corners on "tube," etc.) because my setup works just fine in 1.7 with no glitches.