In the nether i have a double blaze spawner.

When i updated to this snapshot (14W26C) i checked if all my "contraptions" were still functional i found out that blazes spawn in half blocks and on top of spawners that have a 1 high clearance above them.

I added the picture to make it more clear. On the left spawner u see a blaze that spawned in a solid block with his head and on the right spawner u see a blaze that has spawned into a solid block with its feet on top of the spawner.

I hope this helps u make this great game even better.