One of my biggest pet peeves in life is people who proclaim that Wikipedia can't be trusted because anyone can edit it, while at the same time placing their full trust in "professionally-created" encyclopedias—preferably ones printed on paper—put out by the major publishers.
This pet peeve has been on my mind more than usual of late because I've spent a whole lot of the past month up to my eyeballs in reference works about the history of Eastern Europe, and I've found errors in quite a few of them. Two errors stick out for me, because I didn't immediately recognize them as errors and they sent me off on wild-goose chases. Error #1: A book on the history of Poland put out by one of the major library-focused publishers had some of the vital dates for Poland's most famous medieval queen off by about 50 years. (I've returned the book already and I don't remember if it was her birth date or death date or the date she assumed the throne or what, but it was something important like that, and it was WAY off.) Error #2: An entry in one of the major, reputable online encyclopedias listed one of Czechoslovakia's prime minister as an ethnic Slovak when he was actually an ethnic Czech.
Had these entries been in Wikipedia, I probably would have taken the time to fix them. Had they been put out by a company I freelance for, I would have e-mailed one of my contacts there and had them have it fixed. (Well, the online one anyway; there's really no fixing a book that's already been published.) But since neither of those things were true, those errors are going to persist and mislead more people, some of whom will never know that they've been misinformed.
Some errors are unavoidable. People are human and they make mistakes. But the odds of someone recognizing and fixing a mistake go up dramatically as the number of people who have the opportunity to notice and fix the mistake increases. Having worked in reference publishing for 6 years now, I'd estimate that around 4 people really seriously evaluate most things before they're published in reference books. How many people read and edit the average Wikipedia article? I don't know off the top of my head, but I'm guessing that it's a lot more than 4.
That doesn't mean you should uncritically accept anything you see on Wikipedia. Of course there are plenty of errors in Wikipedia as well, both of the "innocent mistakes" variety and the "malicious vandalism" variety. And of course professionally-produced encyclopedias don't have to worry about malicious vandalism in the same way, and that's an important factor to consider when discussing the reliability of Wikipedia. All I'm saying is that the Linux folk who proclaim that "given enough eyeballs, all bugs are shallow" are on to something, and not just in software development.
If you're interested in a scholarly, data-heavy examination of fact-checking in Wikipedia, check out this article.