It's always a cool feeling for me to find a bug in an #opensource library/framework I use. It's extra cool if can track it down and fix it.
For me this is also a signal that I reached a certain seniority as a developer. At the start of my career 100% of issues with other people's code was "holding it wrong". But now I'm tackling harder problems and more often reach the "rough edges" that others haven't cared about before.