You need to cancel a nonrefundable ticket.

What happened: You bought a $98 nonrefundable ticket, but a sudden illness forced you to cancel or postpone your trip. Now the airline is charging you $100 to change your travel dates, plus the difference between what you originally paid and the current, much higher fare.

What the airline should do for you: Nothing. If you want to sit in the cheap seats, you run the risk of paying a hefty fee and a fare increase if your plans change.

Next time: Fly a budget carrier. “Southwest Airlines is the only carrier that doesn’t levy a penalty on changing a nonrefundable ticket,” says travel agent Kathryn Sudeikis. Instead, it applies the ticket’s full value toward a future flight taken within a year of the original departure date. JetBlue charges $20 for changes made online to any ticket, or $25 if done with a reservation agent. Most of the other discount carriers also have low change fees, but the standard carriers charge up to $100 for domestic-flight changes and $200 for international ones. Virtually all airlines will make you pay any fare increases.

Long lines make you miss your flight.

What happened: You arrived at the airport with what you thought was plenty of time, but the check-in and security lines were so long that the plane left you behind.

What the airline should do for you: Airlines routinely advise passengers to get to the airport at least an hour prior to departure for domestic flights and two or three hours before international ones — and that’s all they’re obligated to do. If you don’t arrive early enough, the airline owes you nothing.

Next time: A good travel agent can warn you of potential logjams. And you can find out about about wait times at security checkpoints around the country by visiting waittime.tsa.dhs.gov/index.html. Though the times listed are estimates and averages, not live data, they can give you an idea of the general traffic patterns. Also, when flying from an airport you’ve never used before, leave yourself extra time in case of unexpected obstacles. Getting your boarding pass online or using an automated check-in kiosk will save you time as well. If all else fails, “it certainly doesn’t hurt to let security know that you’re late for your flight,” says travel analyst Jared Blank of Online Travel Review (www.onlinetravelreview.com). You may be allowed to move to the front of the line.