The paperwork that takes longer than you think, the carrier work that has to start weeks out, and an honest answer about whether they should come at all.
Health certificates are valid for days, not months. Some international routes need a vaccination plus a waiting period. People find out three weeks too late.
Cabin or hold, crate dimensions, seasonal embargoes, breed restrictions. Many won't carry flat-faced breeds in the hold at all — for good reason.
A surcharge, a weight limit, one room by the bins, or a rule against leaving the animal alone. All of which you find out at check-in.
An animal forced into a carrier for the first time on travel day starts the journey frightened — and doesn't recover from it during the trip.