I have the rare occasion of having 2 spawners under control in close proximity to each other (maybe 15-20 blocks apart); a zombie spawner and skeleton spawner. The skeleton spawner is located higher than the zombie spawner. I constructed a drop zone with water flow to assist with the mobs dropping out of the opening in the spawner area wall. When the mobs populate significently (say 50 mobs per each containment zone), I am seeing "ghost" zombies apppear in the area between the two containment zones. I cannot kill these ghost zombies. they disappear, and then reappear repeatedly. Additionally, when this is happening, the zombies and/or skeletons drop, and, they then elevate back up to their original point where they dropped from. This repeats frequently.
I would attribute this to an overload on the code with regard to having a very large amount of 2 different mobs in a very small space adjacent to each other.
The drop zone base for the zombies is one level lower than the skeleton's drop zone base. The skeleton drop zone is 19 blocks; the zombie drop zone is 10 blocks.
Whilst not a large negative impact on the gameplay, it is rather unnerving to turn and see a zombie standing right next to you whilst chopping away at the skeletons
Hope this helps.