After finding I could not UNDO a Record Cue (as it was greyed out on the Undo list), I soon found that if I have executed multiple GoToCue's without a regular "UNDOable" command like "1@Full<Enter>", there will be a chain of NON-UNDOables, including Recording and Updating Cues.
This can be an issue if the designer forgets to say "Cue Only" when recording a new cue.
After typing a regular command the grey NON-UNDOables disappear.
I just now noticed multiple Record Cues doing it as well.
I guess the rule for now is always make a level change before updating or recording.
Also, a Microsoft C++ runtime popup appeared on the screen while accessing the UNDO window. It was an assertion failure notice (which, by the way, should only appear on Debug versions of the software).
This was verified on the Offline running 2.0.1