Travel long enough and most of us make this type of mistake.
A few years ago I was flying through London.
Only problem was I didn’t realise until the plane landed . . . . I was flying into Heathrow, and out of Gatwick.
It was lucky I had a transfer time of just over 3 hours, as there are regular direct coach services between the two airports that take around an hour.
If I had tried the train I could have been in trouble as all the trains go into London and then out, so usually take a lot longer than the coach.
Not Just London
London is probably one of the most likely cities to make this mistake as there are four London Airports; Heathrow, Gatwick, City and Stanstead.
There are however many cities with multiple airports.
Even some cities with a single airport have multiple terminals with no direct connections between terminals that can take up to an hour to move between terminals.