Birders are, by nature, a skeptical lot. We're not just talking about woodpeckers here, but just about any rare or unusual bird is questioned until it is seen well by multiple observers, and even then, records commitees are still not always convinced. That's the way the game is played.
Sometimes, though, this innate skepticism can turn around and bite you on the ass.
Case in point: On Saturday, a Chicago-area birder posted that the thought he saw a Black-headed Gull being fed bread at Chicago's most famous birding spot, Montrose Harbor. It was a pretty decent but cautious description; even though the original observer noted a red bill (a pretty darn unusual characteristic for a winter gull in Chicago!), the first reaction of most birders was probably "C'mon, a Black-Headed Gull? Be serious!" And then, the reaction was silence. Nobody went to check out the bird, or if they did, they didn't say anything about it. (We have an excuse--we were in Germany!) Today, a full four days later, there is a post by another birder confirming the Black-headed Gull. Yikes!
How long has the bird been present? How long will it stay? Nobody knows, but there is one thing we know for certain: there will be a lot of BSDs (Birder Sick Days) today in Chicago.
UPDATE: Photos are here: http://www.chicagobirds.com/photos/thumbnails.php?album=70