Some major airlines have made carry-on bag size limits even smaller recently and not everyone is aware of it.

One travel expert who was caught off guard was George Hobica of Airfarewatchdog. He took to USA Today to tell a story of how his carry-on luggage was found to be too big when he was at New York's JFK airport. He said that an American Airlines represtative approached him and "insisted that I put my suitcase in a bag sizer." Hobica says the bag is one that he always uses while flying, but when he put it in the sizer, the bag was too wide. His bag was one inch shorter than the requirement, but since it was too wide, he was forced to go back to check in and check his bag.

Hobica realized his bag failed because American recently changed its carry-on policy to include bags with a maximum width of 14 inches, rather than 15 inches. Hobica's bag was 15 inches. American isn't the only airlines to change its carry-on bag size. Delta, United and American all changed the policy to include a bag with a maximum os 22 inches long, 14 inches wide and 9 inches high.

"So these very specific measurements really change the game, and will be causing a lot of headaches -- and heartaches when people learn they will have to leave their favorite, and in some cases, very expensive, bags in the closet," Hobica said.

Not every airline has such strict limits on bag sizes. Hobica said that airlines like JetBlue and Southwest allow for bigger bags with maximums of 24 inches long, 16 inches wide and 10 inches high, so people can choose to fly with other airlines if they want bigger bags.

There are also carry-ons that are designed to fall within the stricter size limits. Hobica made recommendations of four-wheeled carry-ons such as the Briggs and Riley U122CX Baseline, Travelpro Platinum Magna and CalPak Valley 20-Inch Carry-On Spinner.