I use Cancel buttons for pop-up forms all the time. It usually just hides the containing div, so re-opening the form will show the data as it was entered previ

I use Cancel buttons for pop-up forms all the time. It usually just hides the containing div, so re-opening the form will show the data as it was entered previously. Making the Submit button 2-3x longer than the Cancel button helps mitigate the risk of accidentally hitting the wrong thing.