An unhandled exception due to referencing a null object that caused the Flash app to crash? Developer's fault, not Adobe's.
A crash that brings down the entire Flash runtime (and possibly the browser hosting it)? Adobe(or the browser vendor)'s fault, not the developers.
Even if the developer does something horribly stupid in ActionScript, the runtime should never die due to developer error.
I honestly can't remember ever seeing a Flash crash actually bring down the the browser. Maybe I've been very lucky.
I see that I'm getting down voted for calling 'ED' out on his sweeping statement where he did not define what he meant by a "crash". Note that for many users a "crash" is when what ever App/game/web-page they are running stops with some error and they can't continue. My basic point is that many (most??) of these, in my experience, are down to developer error.
Many developers seem to have an irrational "hate-fest" going against Flash. For sure there's a lot wrong with it but, for me, all to often "its not the tool but the craftsman at fault".