When angry zombie pigmen are chasing you, they will press themselves against walls of closed fence gates. Then if you open the fence gate next to them, they won't walk through it.

To reproduce:
1. Cover both entrances to the lava room of a nether fortress with fence gates.
2. Attack a zombie pigman.
3. Run back inside the fence gates.
4. Wait until a horde of pigmen gather outside the gates.
5. Open one set of gates.
6. The pigmen pressed against the other two gates won't realize that they can get in.