Oily dust bunnies
I was once sent out to inspect a XT PC at a Bus Engineering Depot (where they maintain & repair buses). Apparently the PC had problems printing and odd things happening on the screen. I opened it up and had a look and found it had a inch deep layer of oily dust covering the entire motherboard. I was surprised it was still working at all. As at that time XT's were obsolete, when asked if it was worth repairing I responded, "No, it will be cheaper just to replace it".