This can happen if you have an accelerator defined for a radio button, and then make that radio button invisible. Pressing the accelerator for the invisible radio button hangs the app. The same problem does not afflict checkboxes or normal buttons - it's just a peculiarity of radio buttons.

There's no "fix" as such (it's a bug in Windows itself). The solution is to routinely disable any control which you make invisible, and we've been doing this for *years* because Windows doesn't seem to have any philosophical problem with gleefully handing the focus to invisible windows (d'oh).