The other day I was working in AutoCAD Civil 3D 2011 when, shockingly, it crashed. The interesting part was that it didn't fully crash the program. Instead it just stopped working. Civil 3D was still there. The file was still opened, but nothing would work. Every time I tried to enter a command, in any way, an error message was displayed:
Unknown Command "whatever the command was". Press F1 for Help.