Apparently, what happened was that his pants got ripped. Here's what he said during the MTV interview:
''I have proposed to Katniss. It was good. Actually, funny story: When we shot it, the actual proposal itself is done where you don't hear what we're saying, so I was just saying gibberish while I was proposing to her on one knee. The very first time I went to get down on one knee my pants ripped right down the crotch.
''The worst thing that could happen when you're proposing to somebody is that your pants rip right down the crotch, and that's what happened to Peeta. So... he's doomed. He's doomed. Poor guy, he just can't catch a break.''