I have been checking in when some folks at another clerk paid masses of overweight charges. It is stunning. One woman had 5 extra suitcases! However, the situations that are really surprising people are when they have an oversize bag. You know the sort with wheels that are so huge. A local department store ran an ad for these with the comment 'airline luggage'. If you don't keep up to date on the rules, you might get caught with extra charges. AA is also catching people who are rechecking bags after going through customs off international flights. I heard advice being given at ORD about where the cash machine was!