Continuity mistake: Steve receives a Rowflux and five containers of protein powder. He takes one of the containers leaving only four still on the floor. In a subsequent scene, Steve is still holding the container but now five containers can be seen behind him, giving him six containers altogether.
The 42-Year-Old Virgin - S3-E6
Continuity mistake: This episode reveals Stan has never killed anyone, even though he killed Jay Leno in Stan of Arabia: Part 1, killed Jackson's Double in It's Good To Be Queen, accidentally disintegrates one of his co-workers in I Can't Stan You, and shot a hang glider in An Apocalypse to Remember.
Cock of the Sleepwalk - S9-E11
Continuity mistake: After Stan says "Fired?!" and the camera goes back to Bullock, the word IHOP appears on the board behind them. (00:13:45)
Continuity mistake: Episode 1-10, "All About Steve": The boys are sitting at a table with a Star Trek poster on the wall. Stan walks in and the poster is gone. In a later shot it is back. (00:04:30)
Continuity mistake: When Francine goes to grasp the bottle of wine she was saving, the first one "told Klaus to shut up" has a cobweb on it. When she goes back to get it, there's no cobweb.
It's Good to Be the Queen - S1-E20
Continuity mistake: At the hotel, the dish with the strawberries is sitting alone on the round table. When Stan knocks it off, the table is now empty, but in the next shot, there's a phone and glass on the round table.
Criss-Cross Applesauce: The Ballad of Billy Jesusworth - S11-E17
Continuity mistake: When the game is tied 10-10, Yao Ming and Roger are in the circle at half-court, but when Ming checks the ball back, they are out and in front of the circle and not at half-court anymore. When Roger starts to dribble, he is back at the half-court line, all without them moving.
Continuity mistake: When Hayley closes the van door, Stan and the animals suddenly get a lot closer to the van. (00:08:40)
Continuity mistake: Stan's crocodile skin tie changes back to his ordinary black tie when he says "Prison rules" to himself.
Continuity mistake: When the avalanche happens the second time, all of the animals have snow on their backs. In the shot of all three of the animals, the snow has disappeared. (00:07:30)
Continuity mistake: The poop on the edges of Roger's robe that Stan wears disappears in later shots.
Roger Codger - S1-E6
Continuity mistake: When Francine is telling the kids heaven isn't real the brown chair normally seen isn't there. But in the close up shot of her, the top of the brown chair can be seen behind her. It's gone in the next shot though.
Continuity mistake: In the scene where Francine says she used to be friends with everyone in the block, there is a cutaway of a previous neighbor couple called the McLearys. After the cutaway, keep an eye on Francine's coffee mug. The mug keeps changing position on the table between shots. (00:02:10)
Continuity mistake: In the opening titles (for series 1-3), Stan runs outside and picks up a newspaper. He reads the headline and throws it back on the ground. But by the time Stan drives off, the newspaper has disappeared. (00:00:10)
Continuity mistake: At the convention when we first see Roger and Hayley, they pass a booth that says Costume contest registration. There is also a few people around, a poster and a movable wall. The very next shot is more of a close up of the two and the background has changed dramatically. There is now about 4-5 different stalls behind them and no sign of the previous one or the poster and wall.
Continuity mistake: When Francine is placing a breakfast plate on the table, the arrow on the fridge's temperature dial is pointed to the middle of the yellow. Two shots later, the arrow is pointed to edge of the blue and yellow. (00:00:50)
Continuity mistake: The window partway up the Smith staircase vanishes after the first season.
Continuity mistake: Steve's hair is soaking wet when Francine jumps in the pool, yet dry as a bone when Francine emerges from the pool moments later. (00:03:45)
Continuity mistake: When Francine brings lemonade on a tray for Steve and his friends, she assumes Snot is drowning. Francine then drops the tray and it lands on the ground before jumping in the pool after him. The tray disappears when she emerges from the pool with Snot. (00:03:45)
Roger Codger - S1-E6
Continuity mistake: In the Area 51 flashback, the "RESTRICTED" sign on the lab door goes from red to blue between shots. (00:11:00)
Chosen answer: It is just the decision of the creators to do so.
MasterOfAll