aspiring to do better does not mean aspiring to write bug free code. One is a nearly impossible goal (not over time but certainly at first and certainly when balanced by other constraints)

> About the only thing that annoys me at work is people trying to deny responsibility for their actions (or lack of them.)

There's a difference between denying responsibility and apologizing.

What I do when I find a bug or someone points it out to me is say something like

"I see that. I thought the xxx did yyy but that's clearly not the case. Now that I think about it xxx may also do zzz, is there anything else you can think of that I should take into account"

Or more simply. "Thanks for catching that. I'll put it in the bug tracker. Is this a showstopper for your work?"