I was also notified on startup of a mono crash (that must have happened on the previous shutdown). The only thing I did differently from my normal routine before I shut down was to close, then re-open my laptop lid (which I don't normally do because my wireless card never restarts).
-- Extremely rare mono crash on shutdown https://launchpad.net/bugs/90861 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs