This is a slip-up that was left in the movie because it was fun and didn't really hurt the story. There's a scene where Uncle Billy walks off the screen and you hear a loud crash, and George Baily flashes a big smile and laughs. This wasn't intended, uncle Billy walked into part of the set and knocked it over off camera.
According to Republic Pictures' 50th Anniversary celebration release, the slip up was actually caused by a young film crew member's mistake. As Uncle Billy leaves Harry Bailey's wedding party and exits camera, a film crew member dropped a number of metal containers. Thomas Mitchell (Uncle Billy) improvised off camera the "I'm alright...I'm alright!" line then continued singing. Capra loved the added reality and humor and left the scene in. And, the scene was, in fact, left in the original release. Various edited versions of this classic film exist and like many films, for many different reasons, there are occassionaly editing liberties taken with each release. I recommend the Republic Pictures' 50th Anniversary version. It contains an original movie trailer, the original movie (in its entirety) and a "behind the scenes" documentary hosted by Tom Bosley (Mr. Cunningham on Happy Days) with lots of interesting stories, trivia and facts about the film.
This is not a slip up. There are two versions of this film, the black and white, and the restored colorized version. In the colorized version, the footage that was cut out of the black and white version is restored, and you see the whole scene where a drunk uncle Billy stumbles out of the house after the wedding reception for little brother, asks George for his hat (which is on his head), and stumbles away down the block. The loud crash and bang is where he stumbles into a trash can. I can only guess that the scene was cut because it depicted public drunkennes (frowned upon at the time of the original release)