Continuity: During the part when Rocky, Colt, and Tum Tum are in their room, Tum Tum is playing Nintendo. You can see he is playing Super Mario Land 3, he beats a level and then starts singing Rocky loves Emily. When Colt says "mom's coming" Tum Tum turns off the game but its at the main menu when you first start the game. You can't go to the main menu without resetting or turning the Nintendo off and on.

Continuity: When Rocky and Colt go to the bathroom to attack Hammer after he drank the laxative, Colt thrusts a plunger to Hammer's face. As Colt thrusts it backwards, Hammer's face gets pulled forward, sending his ponytail over his head. In the next shot his hair is neat and tidy, not over his head.

Continuity: In the scene where Colt and Rocky take on the two bullies in a basketball game, Rocky takes a backwards shot from the right side of the basket (obviously shooting towards the left). Then the basketball goes into the basket from the left side.

Other: When the three ninjas are kidnapped and locked in that room, Tum-Tum is complaining about being hungry. Later on, before the fighting match between Grandpa and Snider, Tum-Tum gives Grandpa jelly beans. Why didn't Tum-Tum just eat those before?

Continuity: In the scene where the boys and Emily are riding their bikes to school, the boys and Emily get split up. Emily finds herself running into the bullies and their bikes. The bullies steal her bike. But, if you watch Emily riding her bike before it is stolen, it is a mountain bike, and it is a kind of white/purple colour. When the bullies are taking it away, it is no longer a mountain bike, and it is bright pink.

Deliberate "mistake": There is an obvious stunt double for Grandpa during the fight scenes.

Continuity: In the scene where Emily goes to meet Rocky at his house to ride to school with him, she goes into the house through the door in the garage. When Emily, Rocky, Colt and Tum-tum walk out that same door, the garage is only just opening. If the garage was closed, how did she get in?

Plot hole: Not only does Emily, after being beckoned by Fester (pretending to be Rocky) find it perfectly fine to go out in the middle of the night, she also walks right into their house without knocking. And you wonder why she got captured so easily.

Continuity: When their Grandfather is coming home, his 3 grandsons (his ninjas in training) are hiding in the yard - one of them was in a tree above where he parks his car. The 2 ninjas in the yard come out of their hiding places acting like Ninjas (jokingly) and the one in the tree jumps out onto the ground (about a 15 ft drop...this is one big tree...) and since he's a ninja, he apparently can do this safely. The thing is though, he lands with perfectly straight legs.

Deliberate "mistake": During the sting attempt towards the beginning, Douglas confronts Snyder on the roof of the building. A helicopter rises up from nowhere without making any sound prior to that.

Continuity: When the big Asian guy throws Tum Tum off his neck, Tum Tum is already horizontal. In the next shot, he turns it into a shoulder roll. Impossible.

Continuity: When Snyder and Grandpa fight at the end, and Grandpa's sword is knocked out of his hand, it disappears from the mat and isn't seen again.

Revealing: At one point during the big ninja fight scene at the beginning of the movie, the grandfather grabs the arm of a ninja holding a katana and bends it forward. Not only does the katana bend like rubber here, but after he knocks the ninja out and it falls to the ground, it bounces up and bends even more.

Continuity: When Fester, Marcus, and Hammer first enter the room with Emily as a hostage, it shows Fester holding her with his left arm and his right is holding the gun against her head. When the shot changes, he's holding her with his right arm, with the left holding the gun.

Continuity: During the scene where the main teenage bad guy and his two friends are holding up a convenience store, he gets a phone call from his uncle, about kidnapping the 3 ninjas. His uncle says, "here's the address." But he never actually gives it to him.

Continuity: In the beginning of the movie when Maury and Snider are speaking, Snider at one point whistles for his car or something. In slow motion you can see he never puts his fingers to his lips, he puts his fingers to his chin, obviously he can't whistle so he did some bad acting and faked it.