When my friend Mike, a financial analyst based in Newton, Mass., opened the overhead bin during a British Airways flight from Boston to London several years ago, he got a nasty surprise. No doubt he had listened carefully to the preflight safety announcement, including the bit about "items in the overhead bins tend to shift during flight." (We all listen to those warnings carefully, right? And we all heed them, right?). Unfortunately for Mike, something had indeed shifted. A heavy carry-on bag (not his, but one belonging to another passenger) popped out of the bin and landed squarely on a fellow passenger's head with a sickening thud. Being the courteous and honest chap that he is, Mike apologized profusely and even though the victim insisted that he was fine, he accepted Mike's business card, just in case. Imagine his surprise, then, when a couple of weeks later he was sued for bodily injury. Luckily, Mike's home-insurance policy picked up the tab, which was settled for $20,000. Not for nothing, then, are airlines (mostly foreign carriers) limiting the weight of carry-on bags. Before you jump to the conclusion that airlines are merely trying to force you to check your bag and collect a checked bag fee, consider: Heavy bags can indeed become dangerous when they fall from bins; and the bins themselves are designed to operate properly within specific weight limits. On a recent flight from New York to Frankfurt on Singapore Airlines, the check-in agent asked to weigh my carry-on bag (the only one I was traveling with) and determined that it exceeded the airline's 7-kilogram (about 15 pounds) weight limit. Only my insistence that the bag contained valuable electronics, which the airline would not accept liability for in the case of loss or damage, spared me from checking it. And once on board the Airbus A380, I noticed a manufacturer's sticker inside the overhead bin stating that the maximum load for the entire bin was 50 kilograms (about 110 pounds). And while most U.S.-based airlines and a few international carriers have no carry-on weight restrictions, many do. Hawaiian Airlines is unique among U.S. carriers (for now, anyway) in enforcing a 25-pound weight limit for such bags, while US Airways may insist on checking your bag if it weighs over 40 pounds. I've received numerous e-mails from passengers complaining that Hawaiian's gate personnel have begun weighing carry-on bags at check in and even at the gate, using portable luggage scales, resulting in the imposition of a checked-bag fee. And a large number of foreign-based airlines have strict (and surprisingly low) weight limits for such bags. ANA, for example, limits carry-ons to 10 kilograms (about 22 pounds). Emirates enforces a 15-pound limit; Lufthansa a 17.5-pound limit; and Virgin Atlantic a 13-pound limit. See a chart showing the airlines' weight (and size) limits for carry-ons here. And as more passengers carry on more (and larger) bags, and airlines search for new sources of revenue (and seek to limit their liability), I wouldn't be surprised to see more U.S.-based carriers impose weight limits. That said, not all airlines strictly enforce the rules -- although they do pay attention to the overall dimensions of carry-ons -- but when they do, passengers are placed in the inconvenient situation of removing valuable items that should never be placed in carry-on luggage and finding some other way to carry them on board. And there may be another good reason for limiting the weight of your carry-on. As we age as a nation, hoisting a heavy carry-on into an overhead bin isn't getting any easier. On a flight a few years ago, an elderly passenger asked me to help her lift her roller bag into the bin when the flight attendant determined that it was too heavy to manage ("they don't pay me enough to risk being injured," was her understandable excuse). Being the good Boy Scout that I am, I came to the rescue. The bag did indeed weigh a ton (what was she carrying, gold bricks?). My shoulder took about six months to recover from that good deed, and that was my last attempt at in-flight gallantry.