The only way this would happen would be if the active cooling suddenly stopped working. Otherwise, once the load temperature is reached for the particular component being stressed, say CPU or GPU, the temperature will remain there +/- 3 degrees.
You guys ever folded before? Once you saturate the air in or around your system and it starts heating things up (think a car in the summer) the temps start to slowly rise. This is why server CPUs are binned so high so they can withstand an environment like that if cooling isn't proper to the load. So in other words, if you don't constant cool airflow in and around your case your temps can actually start increasing over the hours. Not to mention the possibility of voltage leak, CPU overwork, ect ect.
folded? I tried doing that but my bandwidth is killing me.. I do run my PC overnight for torrents but those don't do a lot of activity. Does folding do a lot of activity to both processor and memory?
Folding hardly touches internet bandwidth but maxes your CPU and GPU (if capable) in different ways than gaming. It's comparable to say putting IBT and Furmark and leaving it on 24/7.
My PC with the case closed is in my living room with my AC set to 68f even with temps dipping below the 50s right now. My temps still start to slowly rise when I'm folding regardless. This still goes back to my point previously made, if you need absolute stability for something obviously more important than gaming then you don't need to be OCing and should probably purchase a faster CPU. I have my CPU at 4.7 right now as a test (of many things) and I guarantee you that if I put IBT to it I would BSOD or overheat. So far during browsing, movie watching, Skyrim/BF3 gaming, Handbrake encoding, and Photoshop its been perfectly stable. It's a give or take.
Not entirely. You have to be pulling a ton of watts per item to really make your bill go above 5-10 extra dollars a month with one PC.