Already armed with an exhaustive list of checked bag fees, some airlines are stepping up their enforcement of weight limits for carry-ons. This may lead to aggravation if an airline agent weighs your bag at the gate and then charges you.

Although weight limits for carry-on bags are mostly on international flights, George Hobica of Airfarewatchdog.com says charges on domestic flights may not be far behind.

One practical reason for weight restrictions is that they limit an airline’s liability for accidents, such as when a heavy bag falls from an overhead bin and hits someone. But they’re also — you guessed it — a way for carriers to generate more fee income. Under congressional pressure, major U.S. airlines vowed earlier this year not to charge fees for carry-on bags. But they haven’t ruled out widening their restrictions on the weight of those bags.

Although there are practical restrictions on their size, none of the major U.S. airlines puts a cap on the weight of a carry-on bag on domestic flights — but several do on international flights. United, for example, sets a 44-pound limit for coach passengers flying within Asia or the Middle East. American Airlines puts a 50-pound limit on carry-on bags for trips to and from the Caribbean and Latin America. The airlines don’t charge for the first bag on those routes, but you may get stung if your carry-on tops a limit and you’ve already checked one bag. A second checked bag fee could run as much as $60 or $70.

Big international airlines, such as British Airways and Lufthansa, have more widespread weight limits on carry-on luggage. Caps can range from as little as 18 pounds to as much as 50. But most of those airlines also let passengers check one bag for free, lessening the chance that passengers will get stuck paying an unexpected fee.

There are some airlines, however, that have a weight restriction on carry-ons and charge for a first checked bag. Hawaiian Airlines, which flies to the Western U.S. and plans to start flights to New York in June, sets a maximum carry-on weight at 25 pounds. It charges $25 for a first checked bag for flights to and from North America.

While some weight limits seem high, Hobica notes that a bag full of heavy electronics can quickly add up.

Before U.S. airlines would consider implementing weight limits, they’d have to balance the possible backlash: An irate passenger at the gate, holding up the plane’s departure. But other airlines have already put systems in place, including portable bag scales, to catch heavy bags at the gate. So it might not take long for these methods to catch on.

And especially during the holidays, airlines will ask passengers to check more bags as a way to clear the cabin — even if your bags aren’t overweight. They usually won’t charge a fee for this last-minute switch up, but travelers should heed this advice so they can comply with airlines’ requests, while keeping their valuables within reach.

That means you should pack valuables in a smaller bag (like a purse or even a large Ziploc bag) within a carry-on. That way, if you have to check your bag, you can grab your important items and keep them with you on the flight. This will especially pay off if your checked bag is lost or stolen.