When Philip and Emily are eating pizza, Emily is having some sort of vegetable pizza in her close-up shot but Hawaiian pizza in other shots.
When Emily receives a letter regarding her admission to the Youth Symphony, the letter was dated "Wednesday, August 12, 2002". When Emily was recording her message to her friends (who are at camp) early in the movie, she held a calendar for "July 2001". In real life, August 12 fell on a Monday in 2002.
When Gregory was first seen towards the start of the movie, his mother is slimmer when she kisses her husband/Greg's father goodbye. However, when Gregory is seen at Greenbacks stealing candy, his mother is a little plump.