Continuity mistake: In the episode where Lois takes revenge on the four girls who pranked Reese, Lois and Reese use a makeshift catapult to launch paint balloons at one of the girls and her date. The first balloon hits the girl, covering her left shoulder with pink paint. In the shot where her date gets hit with a balloon, her shoulder is no longer covered in paint. Some of the paint blotches change size and shape on both her and her date throughout this scene as well.
Other mistake: When it is Dewey's birthday Malcom grabs a piece of the cake. It goes to Dewey then back to Malcom and he is chewing, but there is no bite in the cake.
Continuity mistake: In the scene where Lois and Hal tell Malcolm he is going to the "special class", Hal notes that the pitcher of iced-tea he makes is always empty, and says, "Look at that - gone," before placing the lid back onto the pitcher. Yet, when it cuts to the wide shot of the table, the pitcher lid is by Hal's elbow. (00:17:50)
Visible crew/equipment: When Malcolm is first visiting Stevie, and Stevie is telling the joke, you can see the hair and hand of a crew member coaching Stevie through the joke.
Visible crew/equipment: In the scene where Stevie is in Malcolm's room. When Stevie is talking you can see the boom mike poking out at the bottom left part of the screen.
Continuity mistake: When Malcolm is in Stevie's room eating cookies, the cookie in Malcolm's right hand disappears and reappears between shots.
Visible crew/equipment: When Malcolm is talking to his first teacher, you can just about see a small part of a microphone at the top of the screen.
Continuity mistake: During the fight between Malcolm and Spath, Spath goes at Malcolm with his right fist, but the closeup of his hand going towards Malcolm and Stevie is his left hand.
Continuity mistake: When the "Krelborne" teacher comes to the house to tell Lois that Malcolm is gifted, Lois comes to the door naked and puts her arm up on the doorway. On the shot from behind she has her hand low (about where her waist is) from the shot from the front she has her hand up high (above her head).
Continuity mistake: During the scene when Lois says that Malcolm is a genius, his arms keep changing position between shots.
Visible crew/equipment: When Dewey yells for Lois because the bow was put on the present, if you look to the left of Lois, you will see a crewmember kneeling down, handing Lois the Red Dress, who is holding a bucket of water. He quickly gets out of the camera's view. [This is on the DVD of the first season. Someone said on the audio commentary that after catching it, they deliberately left it in to see if anyone would spot it. It's still a mistake, but that's why it's there.]
Continuity mistake: When Lois is yelling at the boys for playing with fire, the location of the green scissors on the table keeps changing between shots.
Continuity mistake: When the boys are wrapping their parents present, Dewey is holding a bow in some shots and not holding one in other shots.
Other mistake: Reese is playing a Game Boy without a game in the slot.
Home Alone 4 - S1-E3
Visible crew/equipment: While Malcolm, Reese, Dewey, and Francis are deliberately making their house messy, you can see right above their ceiling a bright light shining down and a black background. It shows that they film their interior of their house on a set.
Home Alone 4 - S1-E3
Visible crew/equipment: When the boys come out of the front door of the house to load into the van to go to the hospital the camera crew reflection is seen on the large picture window beside the front door.
Answer: Francis is only allowed to talk on the phone at school for three minutes a day. I think the code gives him free access to talk to his girlfriend.