When Holly gets into Peter's truck she asks him his name and he replies Peter Lowdel; in the final scene she introduces Peter to Mr. Jennings as Peter Lowell.
The 'mistletoe' in Holly's father's memory box is not mistletoe at all. The leaves are completely the wrong shape and size.
When Peter and Holly's father talk to her at the train station, the falling snow never falls between the actors and the camera, only behind them. It also never lands anywhere, revealing it to be CGI.
When Nick is sitting in a chair reading the newspaper, the headline reads "Manny keeps it going", with a picture of a Red Sox player. Clearly a reference to Manny Ramirez, who played for the Red Sox until being traded in August 2008. This newspaper is from the summer of 2008 before Manny's trade to the LA Dodgers. But in the movie, Nick is reading the newspaper in December. Kudos to the movie staff for at least using a New England paper.