France to Australia - it is quite easy, to mix up as the calendar dates change as you fly. If you think you are landing on "Friday", and that you therefore have to board one day before, then you end up missing your flight that left on "Wednesday".
I once booked the wrong greyhound bus for my DH. He needed to get to work Monday. Wanted to take the Saturday night bus, for the 10 hour trip home, clean up, and be ready for work monday morning.
I kept checking all the routes and finally booked him an express bus that left at 12:06am Saturday night. Except, 12:06am Saturday actually happens on FRIDAY night, don't you know, and he missed it, and the Saturday night bus was sold out of seats all the way to the destination when he went to board just before midnight.