When James Spader lifts the case of champagne bottles at the end of the film, it is obvious that the case has little to no weight in it, meaning the prop bottles are empty.
When Tim goes to help the badly injured Peter on the ski slope, he zips his jacket up all the way (standard procedure for someone who could go into shock). As they talk in subsequent shots before ski patrol arrives, Peter's jacket is unzipped again.
The store clerk's accent appears to change from north eastern to southern from the time Peter meets her to the time she leaves his dorm room.
When James Spader's character visits his ex-girlfriend's townhouse during the holidays, her townhouse front door is the one on the right (in a two-house entrance way). When Cusack's character drops off the case of champagne at the same townhouse later in the film (now that he has moved out and Spader's character has moved in), he drops it off at the door on the left, which is a different townhouse. The error is confirmed when Spader uses the door on the left to enter.
In the parking garage scene, Peter recognizes the car of a senator from Kentucky by the license plate on the front of his car. Kentucky did not issue front plates at any time in the 1980's or 1990's.
When Peter calls Tim (supposedly from NYC) to say he is back in town, Tim invites him to the Stiles New Years Eve party. Tim gives him directions starting in New York, but Peter already said he had to see his aunt and uncle in Hartford, CT first. Tim's directions should have started from there.