That is basically the God fallacy that many engineers fall into. You think because you wrote it, that it has no bugs, and that it's fully understood?
I find it can be highly instructive to run a debugger even on working code, that is not cludge code.
I generally find it doing all kinds of crazy, inefficient things that I probably could not have predicted, even if I'm the one that actually designed and coded it!
Humans are very, very bad at writing robust systems; we never understand our software fully.