When a pufferfish is newly spawned or the world is reloaded, the pufferfish's hitbox box cannot be pushed up against a block like any normal mob. If a pufferfish changes puff states even once, this problem appears to be fixed. However, if the world is quit, then reloaded, the pufferfish will again start acting like its hitbox is bigger than it actually is. The picture on the right shows how pufferfish should interact when pushed, while the picture on the left shows the glitch.