One of the most audacious and famous experiments is known informally as "the door study": an experimenter asks a passerby for directions, but is interrupted by a pair of construction workers carrying an unhinged door, concealing another person whom replaces the experimenter as the door passes. Incredibly, the person giving directions rarely notices they are now talking to a completely different person. This effect was reproduced by Derren Brown on British TV (here's an amateur re-enactment).

I think the response of the passerby is quite reasonable, actually. Confronted with a choice between (a) "the person asking me directions was just spontaneously replaced by somebody different, also asking me directions," and (b) "I just had a brain fart," I'll consciously go for (b) every time, especially considering that I make similar mistakes all the time (confusing people with each other immediately after having encountered them). I know that this is probably not a phenomenon that occurs at the conscious level, but we should expect the unconscious level to be even more automatic.