Up and Down-facing dispencers spawn mobs inside the dispencer block, whereas side-facing dispencers spawn the mobs to the adjacent block. Spawning mobs inside the block can cause them to suffocate and get killed.
Heres a video demonstrating the bug:
http://www.youtube.com/watch?v=ymfFVPisLG4
Duplicate of MC-8293, please use the search function to see if your bug has already been submitted. Currently over 57% of tickets are being closed as duplicate.