Continuity mistake: In the scene where Greg and Rowleys family are playing the game "I love you because..." the spinner they use keeps on changing positions in between shots.
Character mistake: When Greg reads the article about L'il Cutie being cancelled, cartoonist is misspelled cartoonish. The article also repeats the first column word for word in the third column.
Other mistake: When Heather Hills takes a swing at Rodrick with mike stand and hits the chocolate fountain, how does Heather splash chocolate on herself?
Plot hole: When Greg's father finds out that Greg lied about getting a job, one of the workers state that they don't employ minors. However they employed Holly as a tennis coach. She happens to be a "minor".