I have been having this bug a lot lately, and it annoys me, I'm using normal 1.8.8 minecraft on a server. I have a animal pen made out of cobblestone and glass. My coloured sheep keep escaping the pen. its right next to a cocoa bean plantation. chickens, sheep and cows phase out of the walls when they go to the spots where the cocoa beans meet the walls, somehow it eliminates their collision detection and they walk right trough the walls.