It happened to me and many others, and it could happen to you: getting sent to the check in desk with a 20-inch or 21-inch carry-on bag that has never raised an eyebrow with the airlines before. And you could miss your flight as a result. Note: this isn't gate checking. It's back to check in. This only applies to American, Delta, US Airways and United. JetBlue and Southwest have more generous carry-on limits. As does Virgin America, which has a 24 by 10 by 16 (50 inches over all) limit.
I've heard of this going on in Miami, and I experienced myself a couple of weeks ago and saw it happening again last week at New York's JFK in the American Airlines terminal. My 20 by 15 by 8 inch (43 linear inches overall) bag was rejected just before TSA, and even though I arrived at the airport 90 minutes ahead of flight I just made the last boarding call.
I'm only mentioning this because I don't want anyone to miss a flight. So many people get sent back from the pre-TSA line (where a "redjacketed" airport employee checks your boarding pass normally) that the check in lines can be unusually long.
Here's what's happening: the FAA is giving extra scrutiny to American Airlines as they move along with their merger with US Airways to make sure they're enforcing their (meaning American's, not the FAA's or TSA's) 22 by 14 by 9 inch carry on bag limit. And they are enforcing it the letter. So a 22 or 21 or even a 20 inch long bag that happens to be 15, instead of 14, inches deep can be rejected if it sticks outside of the bag sizers placed just before TSA. Again, they are not just checking at the gate (although they check there too of course). You don't get to gate check! You go into the check in line and pray that you make your flight.
Delta and other airlines are also following the bag size rules to the inch, I've learned on Twitter, but for different reasons. It's the busy summer travel season and planes (and overhead bins) are full. One guy with the same sized bag I have (21 by 15 by 8) was made to pay $100 to check his bag on Delta on an international flight.
Just a word of warning.
Interesting, too, that American's carry on allowance info on its website is a bit different from US Airway's but supposedly the two airlines are now following the same rules. As you can see, effective June 11, 2014 US now permits a "soft-sided garment bag up to 51 inches" in lieu of a "carry on bag." No word on exact dimensions for the garment bag. Just 51 inches.
And while we're at it, before travel look up your airline's carry on weight limit too because some foreign carriers have strict weight limits, not just size limits.
And for the record, I'm all for restricting the size of carry on bags. But the recent strict enforcement is something you should prepare for.