Until it is. Swiss Air Flight 111 went down with 229 lives lost because of a new infotainment system whose electrical cables started sparkling.

Yes, not every part of a critical system is critical. Determining which is which is a complicated task, though, as parts interact. Not too long ago (and by this I mean within my lifetime), we were ignorant enough about our own bodies to think that our microbiota was almost irrelevant to our health and that bad teeth were mostly a cosmetic problem.

Back to IT, Heartbleed was caused by addition of a feature that looked as harmless as you could think of. But it wasn't.

Judging real critical-ness of bugs is hard. Yeah, capitalization of an error message is likely harmless, but I would treat, say, any buffer overflow as serious.