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
- duplicates
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MC-8293 Dispensers spawn mobs without offset along the y-axis
- Resolved