Steps to Reproduce:

1. Create a 1 block high, 3 x 3 square
2. Place a pufferfish in water in the middle
3. Place a minecart on a rail right next to the wall

Observed Results:
The pufferfish will expand when you place a minecart on a rail right next to the wall, it will stay this way until you remove the minecart, then it will shrink back to normal size.

Expected Results:
The pufferfish should not expand.