When Allison is on her bed writing a letter to David, she has written four lines on the paper. When we see it, there are five lines.
In the scene when Cassidy and Patrick are in Cassie's room for the first time, Patrick is wearing boxer shorts with a particular pattern. In the next scene after he has read the short story, his boxer shorts are a completely different pattern.
At the very end of theatrical trailer where it show the names of the crew, it says 'Cruel Intentions 2'.