I haven't done the "oops wrong time of day" mistake, however I have done the "oops, wrong airport" mistake.
My girlfriend (ex now) and I decided to hop a flight to LAS and spend the weekend rolling the dice. Well, after booking the flight, packing and calling a cab. We arrived at Chicago's Midway Airport. Upon walking up to the ticket counter and trying to check in, the agent told us that we were leaving out of Chicago's O'Hare Airport.
OH NO!!!!
We quickly jumped in a cab and hotfooted it over to ORD. With 20 minutes to spare, we made the flight.
It was a $95 cab ride.