That moment of suspicion when code works first time.
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It's really satisfying to use a profiler for the first time on a project. I always find a big performance win with only a small code change.
It's never the code that I expected to be slow, however!
Over a sufficiently long time horizon, all code you write is legacy code.
It's weird that GitHub shows time since the last commit, but not time since the first commit. It's a nice way of seeing how mature a project is.