Had an issue where an older Cisco 6500 switch was rebooted, and the MSFC would not start correctly and gave the following errors: "environment checksum in nvram failed" and "rommon nvram area is corrupted".
When trying to log into the the module from CatOS (session 15), the console stated that "module 15 is not installed". Tried to to manually boot the MSFC from ROMMON, using this helpful guide at cisco.com, but kept receiving the "nvram failed" error. I was convinced it was potentially a hardware issue, but we physically removed the MSFC card, plugged it back in, and the nvram message disappeared and the MSFC module booted! Good reminder that sometimes the simplest things can resolve a complex issue.
Note: If you are running a SUP720, there is a field notice for corrupted NVRAM: http://www.cisco.com/c/en/us/support/docs/field-notices/200/fn27595.html