A user-mode application should not be able to cause the OS to crash. If it was crashing, there was a bug in the OS or some kernel-mode driver allowing that to happen. Flash can certainly cause performance issues, hog memory or CPU, and can be the source of security holes, but it can’t cause a modern protected mode OS to crash unless there is some flaw in the OS. At best it might be a trigger for some bug in the OS.
But it certainly is a CPU hog, killing battery life and making the computer sluggish, as well as a never-ending source of security holes. I wish it would just go away, but it’s so pervasive, it seems like we’ll never be rid of it.
OS X didn't crash. . . but Flash would crash Safari, lock it up. Cause other problems. Refuse to let go of the CPU for other applications, even with pre-emptive multitasking. Remember, OS X is UNIX™, the OS very seldom crashed. It just caused OS X more problems than another.