Minecraft: Mobs glitch through glass panes and/or iron bars? Help!

I wanted to collect some spider eyes and ender pearls from my sky mob spawner and because spiders won't drop eyes unless killed by a player I had to separate them and provide a water brake. Problem is, creepers and skeletons are somehow glitching through blocks into the spider chute which has ended badly for me and my base several times now. It's rare but it does happen.

I'll do some testing and see what I can figure out but if you know why this might be happening please let me know.

All I wanted to do was have a temporary solution while I continue work on a subterranean mob spawning/separating system and ran into this quite major issue.