I meant to post this a couple weeks ago on the heels of Bruce Willis's surprise visit to the Ain't-It-Cool News talkback section. If you missed it, a breakdown of what went down is over here. Basically, in response to a long rant about the PG-13ing of Die Hard 4, the mysterious Walter B appeared to alleviate everyone's fears about a castrated John McClane. At first hinting that he "worked on the movie", it quickly became apparent that Walter B was in fact Bruce Willis himself or an imposter (note: his full name is Walter Bruce Willis). Well, it turns out it was the real Bruce. But in the days before this fact was finally confirmed, the talkback was full of rampant speculation and numerous "tests" to prove that Walter B was who he claimed to be. If you've got a lot of time on your hands, the talkback a fairly entertaining read particularly because Bruce Willis hangs around to talk about the movie and other random things in spite of vehement doubters and surly commenters. The funniest part is that Bruce actually fails a lot of the so-called tests to prove his identity, though really how could internet fanboys undeniably prove that they're chatting with Bruce? The answer is iChat, though it took quite a few days for them to figure this out and even though Bruce was openly asking people to join him on iChat, apparently no one on the talkback had a Mac (Bruce is a Mac guy). There was also a fear that they'd come face-to-face with an internet pervert... which is justifiable, I guess.
Some tests that Walter B failed (my interpretation):
The REAL Bruce Willis would know how to spell John McClane and use grammar.
Walter B: Fool, the Real Bruce Willis don't proofread. Proofreading's for suckas!
At the 50th anniversary of the Cannes film festival, what hotel were you in and what could you see outside the window?
Walter B: Hell if I know!
The real Bruce Willis could not possibly forget the premiere of The 5th Element.
Walter B: Who are you to decide what Bruce Willis remembers and doesn't remember?
I was reminded of a great piece by Maciej Ceglowski called Celebrity Turing Test from his site Idle Words, in which he discusses Pixies' frontman Frank Black's visit to his own forums. The article charts a parallel chain of events with Frank Black faced with the same sort of reactions that Bruce Willis would face almost 4 years later... even things such as the moderators stepping in to flatly deny the imposter's claim and then the belated confirmation followed by pages of adulation.
Yet another similar incident occurred with Ben Folds when he visited his fan forums.
Of the top of my head, those are the other incidents that come to mind, but I'm sure there are several other instances of a celebrity dropping in unexpectedly on a fan site and being subjected to random identity tests.