Based on 1.11.2 decompiled using MCP 9.35 rc1; Code analysis potentially outdated
Mob spawners do not test if the entity to spawn collides after calling net.minecraft.entity.EntityLiving.onInitialSpawn(DifficultyInstance, IEntityLivingData) (respectively the methods overriding it). This could result in mobs suffocating if the method onInitialSpawn changed their position.
Note: For naturally spawned mobs this is tested.
- Encase a chicken and place directly above it a repeating command block
- Create a spawner 4 blocks horizontally away
At some point a baby zombie starts riding the chicken even if there is not enough space above causing the zombie to suffocate