We boarded without incident on the nearly-full flight. As we were preparing for departure, the flight attendant did the usual "are you prepared to do the exit-row stuff?" questions for the folks in the exit row, two rows in front of me. The woman in the exit-window seat made some joke about "I'll let him handle it," pointing to the man in the aisle seat. The flight attendant very seriously said, "If you really mean that, I'll have to re-seat you. We're not allowed to joke about this."
The woman allowed as how she wasn't comfortable being responsible for opening a 41-pound (19 kg) door in an emergency, so the flight attendant started to consider how to re-seat her. I raised my hand, caught her eye, and said, "I'll swap. I've no problem lifting a twenty-kilo door." The flight attendant looked relieved, and waved me forward.
So I swapped from my regular aisle seat in a full triplet to an exit-window seat with the center seat empty. The woman complained about moving to a full row and losing the legroom, but she'd brought it upon herself.