While trying to solve a different problem with a report (see: 'Recalc report every time......' for today's date) I had the following wierd thing happen.
I deleted a control whose value expression was just a report variable, vVar. I also deleted the definition of vVar from the list of report variables.
Subsequent attempts to run the report (preview it, actually) failed with the error that vVar was undefined.
I looked high and low in my other controls to see if it was referenced anywhere and it was not. I looked to see if it was referenced by any other report variable, and it was not.
Finally, I did a select all, delete and also deleted every report variable. For all intents and purposes, the report was blank.
Subsequent attempts to preview the report still produced the error.
Go figure!
"The Iron Fish: The water is cold...but the fish don't mind"
...Jay Jenks, boyhood chum