Is it Ever OK to Propose at Someone Else's Wedding?

Picture this: Your wedding day has finally arrived, and you’re at the reception, thanking everyone for sharing this special moment with you. A guest proposes a toast, except instead of concluding with raising his glass, he somehow ends up down on one knee, producing a box from his pocket. Shrieks ensue when he pops it open, and guests realize what’s going on. Be honest – how would you react?