It all depends on the user I guess. For me even prime95 custom settings for extra stress isn't always enough. I have had overclocks that were successfully tested with prime for 24 hours crash over night after I open a few VMs, try rendering a video, folding/mining, etc. Leave your "stable" system with me for a day or two and I'll send it home crying =P.
I remember back when SWTOR first came out. For some reason I was blue screening in that game even though EVERYTHING else was stable. Overclocking is such a pita sometimes.