I recently bought a can of canned herring in mustard sauce. It wasn't past the expiration date; the can was not bulging or leaking, and there were no dents. I opened it up for lunch today and a small amount of mustard - only about a teaspoon, if that much - bubbled out. Neither the fish or the sauce had any signs they were bad, so I ate it anyway. Was the minor bubbling normal, or did I just make a stupid mistake? Mom does home-canning annually, but she's busy at the moment so I can't ask her.