I made a small (about 4x4x4) hole inside of a desert biome and filled it with water, I then summoned 2 dolphins inside (both named "Aqua") and attempted to feed one of them fish. This caused the game to freeze and it wouldn't allow me to save the world. I tried this 3 times and this happened all 3. I suspect this to be because of the dolphin searching for a chest in the empty body of water and being unable to, therefore crashing the game.
EDIT: It seems this bug only happens if I am trying it while using Optifine and not while running the normal game so it is nearly certainly an issue with Optifine and not Minecraft itself