When Kevin is being told about his shoes, in the background the same man walks out of a ballroom twice.
When Pam is talking to Stephanie just before the wedding, the two move relative to the wall from one shot to the next. This is important because Pam tears her veil, as it gets stuck on a splinter on the wall corner.
If you look closely when Jim cuts his tie, you can see that a previous cut was made. The scene took actually seven takes.
As Michael and Dwight are eating breakfast, the waitress in the background is holding the bottom of her coffee pot with her bare hands.