If you try to fill an area which is partly outside of the world (too high or low), it will fail, with the error message "The number you have entered (X) is too small/big, it must be at least/most Y". The command will fail if any part of the selection is outside the world. Instead of failing, it should fill everything it can. This bug causes a lot of problems when executing at entities, and the entity moves close to the void, as it will cause the command to fail.