You maybe on to something. I did change the code as you suggested in your "try something .."
But your sample code that is supposed to duplicate the problem, so far, has not done it. I will keep trying.
Thank you!

>This:
>

>OLDVAL( cControlSource )
>

>will not work IF you are not in the proper area
>

>CLEAR
>CREATE CURSOR bbb (Fld1 C(10))
>CREATE CURSOR aaa (Fld1 C(10))
>APPEND BLANK
>REPLACE Fld1 WITH "aaaa"
>CURSORSETPROP("Buffering", 5,"Aaa")
>REPLACE Fld1 WITH "bbbb"
>SELECT bbb
>? Aaa.Fld1, OLDVAL("Aaa.Fld1")
>

>
>Try something like this
>

>IF !EMPTY( this.control_source )
>    cAlias = GETWORDNUM(this.control_source,1,".")
>    cFld   = GETWORDNUM(this.control_source,2,".")
>
>    * 1. Check to see if cAlias is blank
>    IF EMPTY(cAlias)
>        = MESSAGEBOX("cAlias is empty", 0, "test")
>    ELSE
>        * 2. A different approach to checking if workarea is open
>        IF NOT USED( cAlias )
>            = MESSAGEBOX("Alias "+cAlias+" not open!", 0, "test")
>        ELSE
>            cControlSource = ALLTRIM( this.control_source )
>            IF EVALUATE( cControlSource ) == OLDVAL(cFld, cAlias)
>                RETURN .T.
>            ENDIF 
>        ENDIF
>    ENDIF
>ENDIF
>

>
>
>>Yesterday I made the change to the code, as was suggested by John Ryan. I also set TRY / CATCH around the entire code. This will allow me to catch an error, if it happens and save it into a hidden log. I installed this code at 6 customers PCs. I will be looking at the hidden log from time to time to see if the problem occurs.
>>Thank you.