>Thanks, but I am really wondering if there is some way that I can configure VFP to error out when such a condition arises. The last thing I want is for such a dialog box to pop up at a customer site. I would much prefer to trap this condition and deal with it apropriately.

It doesn't seem to be trappable; no error gets handed to the VFP error handler for the couple of examples I tried unless you fail to respond to the OPEN dialog, and I don't see somewhere that you could cause it to function differently. You'd have to encapsulte your offending code to check if there is currently a file in use in the current work area, something like:

IF USED()
  LOCATE FOR... && or whatever else you want that needs a table open in the CWD
ELSE
  *  nMyIDidSomethingDumbError is an integer corresponding to the value
  *  given to the error handler;  if VFP has an error that it handles the
  *  way you want (like shutting down), fine, otherwise, add code to your
  *  error handler to deal with this when this error code is seen
  ERROR nMyIDidSomethingDumbError  && Hand the error off to your error handler
ENDIF

>
>> You must be using functions like locate, seek etc., without opening necessary table. Check your code for this using debugger