When a fence gate is powered by redstone, it sometimes renders as open, but really isn't. When clicked on, it does truely open.

How to replicate
1. Place a pressure plate next to a fence gate.
2. Stand on said pressure plate.
3. Click on said fence gate.
4. It renders open, but in fact, is closed.

Alternative Method
1. It happens randomly when walking over said pressure plate, into said fence gate.

Update:
While bugged, the fence gate does not get affected by redstone to truely open.

How mobs are affected
The mob AI thinks it can walk through the bugged gate.