There is a tutorial describing how to build a mob farm. It uses following pattern:
S = Stone (or other solid opaque block)
W = water 2 levels under the stone
H = hole
B = border wall
|--------------20 blocks--------------|
B B B B B B B B B B B B B B B B B B B B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B W W W W W W W W H H W W W W W W W W B
B W W W W W W W W H H W W W W W W W W B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B S S S S S S S S W W S S S S S S S S B
B B B B B B B B B B B B B B B B B B B B
When the farm was built, mobs started to fall out immediatelly. This is a little suprising. If they appear on the stone platforms, shouldn't they just wander and only randomly fall in the water streams? How come they fall in so frequently? Do they walk there on purpose?