I thought the same many, many years ago when I modified my old escort rally car fuel tank to move the fuel outlet away from just behind the rear wheel. Half an hour flushing through with water, then welded a plate over the exit hole.
Near the end, I accidentally blew a hole and there was a quick high pitched pop and I heard a rustle in the nearby hedge. When I'd finished, I realised the filler cap was missing - I found it under the hedge where I'd heard the rustle. I then went to refit the tank and found it was now too big to fit back where it had been, especially the flat sides which were now curved outwards like a ballon.