Went to LHR T1 for a short-haul, queued for 30 mins for bag drop, only to be told by a very polite BA guy that I was going from T4. Should have known to check....
Had to absolutely leg it to check in to make it, that's where travelling a lot helped...
And more recently, turned up at the wrong HI in London and couldn't remember what one I had booked (there were 3 in the area), didn't have a print out and couldn't access my emails. Fortunately the concierge let me use his computer to check my webmail - in fact staff at that hotel were great and so friendly about it. And then had a lovely taxi driver who gave me a discount too because it was a short distance, but late at night, and I made him laugh a lot about my stupidity.... So not all bad, because I saw the good side of some people as a result!
But you are left feeling stupid, such silly basic errors