I was wondering if it was in a lacrosse game. They had him watching a game, wistfully and imaging back to himself playing.
They showed it in the final episode of the season, the one that introduced the (gorgeous) Sela Ward. When it airs again, run tape on it; it's one of the best-written and performed hours of TV I've ever seen. It starts with House being forced to teach a class on forensic medicine, turns into hypothetical cases of three people, all with different leg injuries, then at the end, you discover he's been spinning these stories from his own experience and you see how he became limp and what Sela Ward and his own stubbornness had to do with it. Absolutely brilliant.
The idea that that dried-up Clinton fantasy "The West Wing" was nominated for best drama series Emmy AGAIN and "House" wasn't shows you how moronic and politically-partisan most people in the TV industry are.