It's dead easy - just a combination of circumstances. As a Heathrow controller I've personal experience of:
a) A/c mistaking Northolt for Heathrow and ATC insisting that they turn left a bit!
b) A foreign 707, when told "12 miles from touchdown on 09L", having been turned onto the ILS at 4000 ft replied, no we're 150 miles from touchdown; our destination is Manchester".
c) A foreign DC8 landed at Heathrow and as he tirned off the runway said "!!!!!; we're supposed to be at Gatwick".