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!