I've seen a few different mob spawner farm designs around. Lots of people seem to like this one:
The idea, as far as I understand, is that mobs spawn in the air and fall down through the water suspended by signs. They then get sucked into the "funnel", pushed over to the water ladder (which they climb by swimming up), then dropped to near-death.
I've seen farms like this in operation in SMP, so I understand the basic principle. However, I attempted to build one from this schematic, and it doesn't seem to be working. It seems that the mobs are able to float / swim up on the water blocks below the spawner (above the signs), meaning they never actually fall to the bottom to get sucked up the latter.
As so many people seem to like this design, I'm willing to bet that I made a mistake. However, looking at the schematic, I see two different places -- the aforementioned water and the water block at the beginning of the "funnel" -- where mobs could get stuck. How is this farm supposed to work, then?