You feel bad that the waiter doesn't understand your much-practised Dutch. And then you realise you're in Hungary.
You start to refer to all foreign currencies as 'Dinar'. "How much is that?" "400 Czech Dinar".
You have to arrive at the airport 3 hours early because you've forgotten what time zone you're in.
You instinctively grab your passport when you're going to meet a friend just out of town.
You hold sweepstakes with your seatmate over how many minutes you're going to be holding at Lambourn.
By spotting Clapham Common out the window, you know exactly the approach you're going to take into Heathrow, and how long it will take to taxi into the gate.
Driving to Schiphol, you look at the planes departing, and hope they're going on the Zwanenburgbaan.
You know exactly where to stand on the Heathrow Express platform to be first on into the carriage that lets you off for optimal positioning to be first in the taxi queue at Paddington.
OK, some of these are a little specific, but still...