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.