It's true that merely the ability for widespread code inspection doesn't mean all the code really gets widespread inspection [although I'm surprised by the number of messages I see on mailing lists like Q: "Hi, I'm a Chinese grad student and have been reading the gcc source... I don't understand how XXX can work, given that YYY... can you explain? thanks" A: "oh, hmm, actually, that seems to be a bug..."]

Still, I think a common pattern is (1) notice funny symptom, (2) go look at code, puzzle through it for a while, and then "oh!" You're now in a much stronger position to fix the problem or petition for a fix.

With closed-source code, step (2) is a lot harder unless you're in a privileged position...