In VFP 7.0 with Service Pack 1 installed I created a class named Test as a textbox and added a property named testing that is hidden. In the class' valid event I put this code;

This.Testing = "ABC"

Then I created a form and put one of these textboxes in it. I added the following code to its Valid event;

wait window "fired" nowait
DoDefault()

I ran the form and tabbed out of the textbox. I saw the wait window and got no errors.

What am I doing differently than you?