At work we use NUnit.Extensions.Forms.ButtonTester and its siblings to unittest forms.
This works nicely, however I ran into a problem where NUnit complained that the button was not visible, but it actually was!
Well of course I googled for this problem, and found only four answers, three of the pages I could find, were not about the exception as far as I could see.
The third page was about this, however the page was gone (HTTP 404), so I found the answer to my question on a google cached page...
Since the information is so inaccessible I thought I would make it available here:
If a control is on a tab or the like, you might get the above mentioned exception eventhough the control is visible.
The workaround I found was to fire the event like this:
checkBoxTester.FireEvent("Click");
Note this is for NUnit 2.4.7