Click the green flag. Click the blue circle with the check box on the prompt from the Ask Block. The glitch is that the mouse click to the button goes through and is detected by the stage. The click does not register on the stage if you click other parts of the prompt. This happens in both full screen and normal mode. It does not matter if you enter anything at the prompt or not.
Also take a look at this possibly related post in the Bugs and Glitches forum: http://scratch.mit.edu/discuss/topic/84476/
