I was just terribly embarrassed. I know I did 'Undo > Mark' prior to a particular experiment on a user's drawing to find a solution to a problem. I use this all the time... The solution worked but the user wanted to go back to the 'Mark'... so I did 'Undo > Back'...
I was surprised to see it undid all the way back to a point beginning.
I know that if there is not a 'Mark' placed, then doing Undo > Back would prompt 'Undo > Back'... 'This will undo everything, OK? <y>'...
But is there a setting in Undo that could have fouled up the 'Mark' I placed prior to the experiment?