Hello
I had a similar problem where the error would occur if the keyboard shortcut for the command button was used.
I was using the On Key Command for the shortcut which appeared to fire outside the firing order of the windows objects on my form, so the Valid event of the text box my cursor was on would fire AFTER the command button code which had a Setfocus() within the code.
If this is your case, try using the Forms own KeyPreview Property = .T. and interrogate the keystrokes there.
Hope this helps
Stuart Ramsey