OS upgrades of any kind (MS or MAC), verify your system can handle the upgrade, verify you have or get install media for everything currently on your system, backup (image backup that you can also restore individual files from is best), scan the disk for issues or problems including the unused portions (MAC verify/repair permissions as well), do upgrade (MAC, using "combo" update file), then test everything ASAP. If issues that can't be resolved, do clean install. This is not rocket science but I do sympathize with the folks caught by failed upgrades.