This problem has happened to me TWICE, and all though it isn't very concerning, I still want it fixed because it's ruined some of my worlds. The problem is that mobs are escaping from areas that are IMPOSSIBLE to escape from. Here are both situations:

The Cat

First I set up a world where there was a wooden door in the house. The villagers would open the door, but they couldn't get out because there was a gate on the outside, and villagers can't open gates. So anyway, I had a cat in that house, and he OBVIOUSLY couldn't open the gate. But the most unbelievable thing ever happened. I was checking on a building right next to the one the cat lived in, and to my surprise, the cat appeared next to me (it was for sure the right cat because the name tag was above it's head.) and meowed. I immediately put him on a lead and brought him into the house, and after a couple times of this happening, I decided to attach him to a fence with the lead.

The Prisoners

I built a prison. It had walls made of prismarine, and iron doors with pressure plates let you inside. On the inside stood three cells, all with iron doors and prismarine walls surrounding them. On the outside, three pressure plates were in front of the cells, one for each, but none on the inside, where one villager per each cell stood. Each had a name tag above them. I checked on them, and everything was good. I used cheats to make it day time, and checked back in. They were gone. I ran to the nearest village where one of the prisoners had gotten a job, and the other two were wandering outside of the prison. There was nothing to let them escape.