>>Did You have somewhere _screen.LockScreen = .t.?
>
>Nowhere in either form actually

O! I now realise tha You made a ShowError form Modeless for testing. So the execution of the program didn't stop after DO FORM..
i.e.

.Visible  = .f.
DO FORM ShowError ....
.Visible = .t.
&& This line is executed immidiately after DO FORM ShowError and because the ShowError is not modal VFP didn't stop execution of
&& this code until the form being closed. So the main form just flashes :o)))

You can test a littile
Make a ShowError form Modeless again and do:

.Visible  = .f.
InKey(0)
DO FORM ShowError ....
InKey(0)
.Visible = .t.

to see what happens