I found a fun but serious SEO mistake in a Google Webmaster Help thread that I wanted to share with you all. But I felt it would be fun, if you had the time, to share at least one of your funny, but deadly, SEO mistakes - that you have seen first hand, in the comment section below.
The mistake documented in the thread was that someone was not sure why his Webmaster Tools account was showing the status of "network unreachable" for many of his URLs. JohnMu from Google looked into the issue to explain that although you and I (ordinary users) can see those URLs, for some reason, GoogleBot was being shown a 500 error page. The way John was able to reproduce that was by simply changing his browser's useragent to the same useragent GoogleBot uses, "Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)". Then he surfed to the page to be presented with the 500 error.
This is a deadly but sad mistake. Normally people will go out of their way to serve up better optimized pages for GoogleBot. But to serve up error pages to GoogleBot, while serving up your beautiful content only to your users - well, that is kinda anti-SEO. This webmaster's issue had to do with a problem with ASP.net's URL rewriting system he was using and how he set it up.
Your turn, share a quick story below of a funny SEO mistake you have seen.
Forum discussion at Google Webmaster Help.