While exploring a swamp biome, I encountered a Desert Temple in the middle of a shallow lake of water. Pressing F3 confirmed that I was in a swamp biome. Inside and on top of the pyramid were approximately three witches. I'm not sure if it's plausible, but it may have generated the temple accidentally instead of the hut. I have so far only encountered this once, and other witch huts have spawned without concern.
EDIT: I tried the seed again (-6801647526377601160) and got the same result. The center of the pyramid is located at (698, 65, -645). The witches only spawn on the pyramid at night.