>>>Hi, I have a TextBox with a ControlSource that doesn't exist - no table is open; there is no table in the DataEnvironment, Form.Load() and Form.Init() are empty, and the form is not based on any class other than "Form" itself.
>>>
>>>And yet, instead of getting the expected error message about the non-existant field, the data I type in mysteriously gets saved - when I get back to the form, it is right there.
>>>
>>>Is this something with the Registry that I didn't know about?
>>>
>>>I just type some text as the ControlSource; for example, "aaa".
>>
>>Did you try closing VFP and re-opening? If you do so, are the data you typed still there?
>
>Indeed, the data disappears after closing VFP.

Ok, so I suspect that a private variable being created that holds your data.

You can add

=messagebox(vartype(aaa))

in the form's Init.