Mobs will escape from fences. I don't know how it works but i think it is when mobs are next to fences and the chunks unload and reload.
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I have this same issue, but they don't seem to be 'glitching' but rather literally jumping over the fence (I've seen them do it) It happens most easily while holding lurefood (what every lures them) and rabbits seem to be able to do it the easiest. Sometimes when they jump on the fence they start spinning around until they get nudged.
I'm using Version 1.8.3 and Java 1.8.0_25 (x86-64 : aka 64bit PC)
Dupe of MC-2025