A frequent response to examples of mistakes in Wikipedia is that "mistakes get reverted quickly." If I were to write "Barack Obama is a Klingon from the Classic Trek era born in the year 2288 and sent back in time to fight the machines" in the Obama article, that would get reverted within the minute, if not sooner.
But that's a blatantly obvious error and no one would take it seriously even if it wasn't reverted immediately. What if instead I put in a mistake about someone "notable" enough to merit their own Wikipedia entry but not famous enough to have more than 30 'watchers'? (Watchers are Wikipedia users who put the article on their watchlists to be alerted of changes in real time).
Gregory Kohs at Akahele posted a carefully-researched account of how the Mike Ilitch article was wrong for almost three years! (See http://akahele.org/2009/07/where-in-the-world-was-mike-ilitch/). Randomly pick a date within the past six years the article has been up, and the chance of Ilitch's place of birth being correctly stated is about the same as is that of a tossed coin landing on heads—50/50.
Don't even get me started on the Seigenthaler scandal, the false Wikipedia biography that stood for months. If his son hadn't noticed it, who knows how much longer it would've gone unchallenged?
To my knowledge, the most recent example of an inexcusable Wikipedia mistake is the Sarah Palin death rumor I mentioned some posts ago. I had already mentioned that although that stood for just little over half an hour, it is still inexcusable because the article has almost seven hundred watchers. Sarah Palin was unknown five years ago and will hopefully sink back to obscurity in another five years, but at this point in time, how do you explain that none of the almost seven hundred watchers were awake and logged on at the very moment the false death announcement was made?
Maybe you think you're helping Wikipedia out when you correct a mistake you detect in it. Next time you do that, after your edit, look in the edit history (by clicking the "history" tab) and try to determine how long ago the mistake you just fixed was inserted. The results will be quite enlightening.