Hello Marcia !
Sorry for my English, probably I should have written "I was advised". Actually I was advised by Nadya Nosonovsky try to put DoDefalt() inside the InteractiveChange. But it it not very important.
My question is: why the value of the control (Combobox) changed but the InteractiveChange Event doesn't seem work ?
You wrote: "Yes, I agree. It is probably a bug in your code < s >."
But there is no much code in InteractiveChange(), just one line

ThisForm.Button1.Enabled=.T.
(or if you don't like it This.Parent.cmdDoSomethingSpecial.Enabled=.T.)

As I said before, i'm not using something special , just create a new form, put standard ComboBox & Button and that's it. In Init() method make This.Parent.cmdDoSomethingSpecial.Enabled=.F. Want to make this button accessible when user chooses something, so put in InteractiveChange just one line. I don't see a bug in this line.
Of course I know I can do validation, check empty value and so on, but why it happens ? As I mentioned before it happens once in while, but it does.

Thank you very much, Mariam