As Menchar and Roberto have pointed out, the most likely scenario is that your Quit button is the active control at the time that the Enter key is pressed. You could confirm this by adding a GetActiveCtrl call to your code as you quit your program (but before you discard the panel).
Earlier you mentioned that the Quit button does not have the outline drawn around it to indicate that it's active. But that's a not a foolproof way of indicating whether it's active. If, for example, your UI is not processing events, the control might not have had a chance yet to update its look after changes in your program.
You also mentioned that you were activating a different control at the beginning of your control. But if the control that you're activating is dimmed at the time, or is dimmed after you activate it, then the focus will necessarily change to another control, possibly your Quit button. Finally, if the Quit button is the only non-indicator, non-dimmed control in the panel, then it necessarily has to have the focus.
Note that even when the Quit button has the focus, if the user presses the Enter key, the button's callback does receive an EVENT_KEYPRESS. You can, if you wish, respond to this event and return 1 from the callback function, which will prevent the control from then receiving the EVENT_COMMIT.
Luis
