Jon's fascination with libraries has given him a deep understanding of the ISBN format. This reminds me of all the people that "whip" together a XML parser and forget to support all the complex corners of it. Everything is much more complicated than it first seems once you have understood it completely.

Favorite bug of the day: Someone was parsing version numbers and thought the format was always 4 sets of 1-4 digits separated by periods. (1.23.456.5678)... of course, the actual format for version numbers is 4 DWORDs, which, when printed can be many more characters long... boy I wish people would read specs before writing code!