One of my users just discovered the following:
If you hit ESC twice when on a page containing an update panel with a control inside, if you click on anything else that does a postback, you'll get a System.ArgumentException: Invalid postback or callback argument. Event validation is enabled using [...] error.
I would like to know more about the default behaviour of the ESC key. And might be interested in hooking up to the event to fire something else when that happens.