One time I was working on a MapReduce that processed a lot of XML found on the internet (I can't remember why anymore (edit: I remember now why I can't remember, it was a friend's program that I was helping out on)) and I found it crashing on some input. After some examination of my code I traced it to a bug in libxml (which is also used by Chrome, Safari, and others). I simply reported the bug to the appropriate parties and it got fixed. It's funny to think that the author of that bogus xml file had gotten the syntax wrong enough in a way that would've been worth thousands of dollars!