Mobs on grids?

[ReFro]

I have about 14 mobs on a grid, the terrain is a little hilly. When they start walking, they start dropping outta the sky pause for a sec and continue on there merry way.
My ? is, is there any way to get them to stop falling out of the sky?

[sotonin]

This is a known bug, it happens less than it used too. but still occurs.