This is a repeat of a reply I offered to a sad soul and thought it to be important enough to post a new thread so that everyone might benefit.
This doesn't have to happen. Support the flat sided filler spout with a 16mm open wrench and turn against the 16 with a 14mm on the plug's hex head. That way you are neutralizing the stress placed on the thin sides of the pot metal fluid reservoir. Replace the plug with a brass or nylon washer and just snug it down. End of story.