With System 7 or Mac OS 8/8.5/9, if one used it for long enough with a stable software setup you'd eventually get a gut feel for what programs, extension sets, etc were most likely to invite a crash (it wasn't a terrible idea to reboot after a long web browsing session with Netscape for instance). It wasn't surefire, but one could get it into a somewhat stable state. You never stopped hammering ⌘S, though.

Windows 2000 was incredible. Running it after having wrestled with 98SE was like getting teleported from a garbage dump to sunny meadow with a fresh ocean breeze. I've never seen machines transform quite as radically as they did when upgrading from something earlier to 2000.