7/28) If you look at the clocks in the diner, they're all stopped, mirroring Phil's predicament.
8/28) There are exactly 38 days depicted in this film, either partially or in full.
9/28) Harold Ramis considered Chevy Chase, John Travolta, Steve Martin, and Tom Hanks for the lead role, but he considered them all 'way too nice' compared to Bill Murray.
10/28) A scene was shot in which Phil destroys his room, slashing pillows, spray-painting the walls, etc. He also shaves his head then the camera pulls back from his face to show that his hair and the room were back to normal the next morning.
Harold Ramis had trouble making the dissolving shot match so the scene was changed to Phil breaking a pencil instead.
11/28) Phil at the piano teacher's house, when he is fumblingly playing Sergei Rachmaninoff's 'Rhapsody on a Theme by Paginini', is actually Bill Murray playing. He does not read music, but he learned that much of the song by ear.
12/28) Bill Murray was offered a spit bucket for the diner scene where he gorges himself on pastries but he refused. He admitted to getting sick after having too much angel food cake.
13/28) Whenever Harold Ramis was explaining how he wanted Bill Murray to play a scene, Murray would interrupt him and ask, "Just tell me - good Phil or bad Phil?"
14/28) On the DVD, Harold Ramis states that the original idea was for him to live February 2nd for about 10,000 years.