Agreeability with RAM is often a huge issue which won't always be explicitly reported. Also add-in cards - some of my machines are super-sensitive to even the slightest mis-seating in the slots. (The same problem plagued Nubus too and often worse as oxidation and loose contacts set in over the years.) Even with perfectly stable hardware, however, the Mac OS seems to perpetually corrupt files or file attributes. Unless these things are constantly being scanned for and corrected they will eventually ruin the whole operation. I don't think I've ever had a Norton scan (or similar product) more than a few days apart not find at least a few minor errors. I could never understand how this happened so consistently and progressively to any drive throughout the entire life of the Classic OS. OS X isn't much better is this regard though.