Audio problem: In the mansion, the boys open a secret passage and fall through the floor. They scream after shining a flashlight on a skeleton. Eugene then can be heard whining that he wants to go home, but his mouth is not moving.

Plot hole: The army showing up at the end of the movie is wrong on numerous levels. 1) Eugene wrote and sent out the letter that day. I find it hard to believe the mailed letter got to the army in just a few hours time. 2) How did the army know to show up at time square? When Eugene wrote the letter, nowhere did it say where to meet. Plus the boys didn't decide to go to time square until the last minute. 3) since when is the army deployed to battle monsters based on a letter that was written by a child?

Deliberate "mistake": When the vortex is summoned to get rid of the monsters, other things get sucked into it. Cars, trees, light poles, other police officers. however other things or people who are more accessible do not get sucked in. For example, Phoebe hiding behind a bench, the bench itself, Phoebe's mother and a few cops hiding behind a cop car, etc.

Continuity: As the vortex is going through, Frankenstein's monster is saying goodbye to Phoebe, while Phoebe is pleading with him to stay. At one point Phoebe is holding Frankenstein's monster with both hands. Then after he gets sucked into the vortex, she throws him her stuffed dog. How can she hold the monster with both hand and still keep the stuffed dog without it going into the vortex?

Continuity: When fat kid is confronted by the Gillman and has picked up the shotgun, he looks inside and sees E.J. and another kid hiding behind the counter. Fat kid begs them to let him in. E.J. and the other kid shake their heads no and lower themselves behind the counter. A few seconds later, a distant shot shows the two shaking their heads and lowering themselves behind the counter again.

Continuity: Phoebe is on the left side of the park bench as she watches Frankenstein fly into the portal. Just before it closes, you see Phoebe move to the very left side behind the bench. When it cuts back to the close-up of her crying, she's back where she originally was.

Plot hole: In order to get rid of the monsters, Phoebe needs to read a text from Van Helsing's diary in order to summon the vortex. As Dracula approaches her, she becomes scared and misses on some words (the Scary German Guy is helping her read since she is 5 years old and she doesn't repeat some of the words he says). However the vortex still shows up to get rid of the monsters.

Continuity: When the monster squad first encounter the Mummy, Sean mentions it is 20 minutes till midnight. A few other scenes happen next before the boys arrive at Times Square. However, the clock in Times Square shows only a few minutes have passed.

Other: The werewolf gets blown to pieces by the dynamite, clothes and all. I can understand how the werewolf can reassemble his body as only silver can kill him. But how in the world do his clothes knit themselves back together?