Maybe no-one would have noticed if it was closed source. I bet if Microsoft released everything as Open Source there would be billions of bugs discovered.
The Debian RNG bug was noticed by folks who found identical certificates in the wild, not by code inspection. Similar RNG weaknesses are commonly found in closed systems as well, so it doesn't seem to be a particularly open/closed source thing.
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...
