Goofs

At Heather Hills' Sweet 16 party, during LD's performance, Heather Hills swings at Rodrick with his mic stand, missing him, but in turn knocks her chocolate fountain over & away from her. In the next shot, she's seen standing next to her friend on the opposite side of the fountain, without the mic stand, & the chocolate fountain has been splattered onto her somehow.

When Greg gets caught lying about being employed at the country club, the manager states "We don't employ minors", despite the fact that Heather Hills, who is planning her sweet 16, works there as a lifeguard.

When Susan is telling Frank about Manny's lost Tingy, Frank replies with "I'm sure it'll turn up" while facing forward talking to Susan. In the next shot Susan is behind him finding the Tingy.

When Greg and Rowley bump into Holly at the country club and she tells them she is teaching tennis there, a couple sitting behind them gets up from their table to leave. After cutting away to Holly and then going back to Greg and Rowley, the same couple can be seen getting up from the table to leave a second time.

When Rodrick hands Greg the remote control for the pyrotechnics at Heather Hills' birthday party, he is wearing a sleeveless shirt. In the next shot, he is wearing an over-shirt which he takes off to reveal the sleeveless shirt.

The characters played by Joshua J. Ballard and Matt Mazur are swapped in the end credits. In fact Ballard was Spag Union Tracking Device Boy and Mazur was Spag Union Math Boy.

When Frank Heffley checks his watch and says that he's late for work, he actually looks at his left arm, which doesn't have a watch on it. However, this was deliberate and not a mistake, as Frank needed an excuse to leave and so didn't actually bother to check the time properly.

Although the movie is set during the summer holidays, fall foliage is clearly visible throughout the film.

At Heather Hills's Sweet Sixteen party, when Rodrick is singing, he accidentally bumps into the large ice bust of Heather and knocks it to the floor. Normally, if an ice sculpture is knocked over, it would shatter and send pieces of ice everywhere. However, while an audible shattering noise is heard, the bust simply bounces when it hits the ground, indicating it is clearly not made of ice.

When Rodrick is singing a song for Heather's Sweet 16, and Rodrick hits the ice sculpture, a lighting crew is seen on top of a table in the background.

In the scene where the tennis ball falls on Greg's face, a shadow of a hand can be seen on the very top right corner of the screen. Depending on what screen you have, you may or may not see it. You won't be able to see it on a TV airing because they usually crop the image to fit the TV.

When the Cranium shaker moves upwards, we see some mountain scenery and movie studio buildings below, and not the usual boardwalk scenery where the movie should take place.

Holly asks Greg why he didn't call her, despite the fact that she knew she never got to finish giving him her number.