Before Captin Von Trapp sings Edelweiss (after Lonely Goatherd) he is holding his glass in his right hand and after they vote him to sing he has the glass in his left hand.
@ssKicker, in response to the comment on switching glasses, the scene changes for only a second or two and his glass is changed. If he had changed hands I would've seen.
Celinecjc84: what is the point in signing up or even visiting this site if you have no interest in slip-ups? The whole point of it is to catch mistakes...if you're so interested in watcing the movie, why are you on this site?