Witch Huts generate in Swamp biomes regardless of other biomes intersecting their structure - most commonly, a part of the Witch Hut is intersected by a River biome cutting through the swamp.

This is problematic because Witch Spawning is not only limited to the small area of the Hut, but can additionally only happen in the corresponding Swamp biome, which means that the blocks of the structure that are not inside a Swamp biome will not spawn Witches, but any other hostile mob that normally spawns if the block meets the conditions necessary.

This is especially annoying when trying to build a Witch Farm out of the Hut, since is greatly limits the number of Huts on a map that you can make a 100% efficient farm out of.

The screenshots attached show the problem with an example Witch Hut. The roof has been replaced with Wool, and the top view shows very well how many Blocks are actually able to spawn Witches (green) and how much potential spawning space is taken away by the River biome (red).

The problem is also very well explained in this video by docm77:

http://www.youtube.com/watch?v=KllWkdfOS3M

Starting at 1:25, he explains how small the actual spawning area of Witch hut is while he later elaborates on the River biome problem - and I have found many examples on the server I play on of Witch huts being more than 50% "ruined" by River biomes (or in one case, a Desert, which creates essentially the same problem).