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: 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.
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.
Continuity mistake: After the last school day has ended, Stan chases Steve into a canyon Terminator 2 style to show him camp brochures. After Steve has a go at Stan about not wanting to go to camp, Francine enters the scene, knocking Stan's car out of the way. The car is almost ten meters away from Steve and Stan as she enters the scene. Francine doesn't move the car in the next shot. But in the next shot after that, the car is suddenly right next to Steve and Stan. (00:01:15)
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.
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.
Phantom of the Telethon - S4-E7
Continuity mistake: The way Greg holds the volleyball at the start changes instantly. (00:02:00)
Phantom of the Telethon - S4-E7
Continuity mistake: When Stan comes in and says that Bullock loved his telethon idea, Roger goes from holding his wine glass to having his arms crossed depending on the shot. (00:06:30)
Phantom of the Telethon - S4-E7
Continuity mistake: After Roger gets his organ, we see Stan by the totals display. However, the donations jump from 382,760 to 382,840 between camera cuts. When we see the donations rise normally, you see the figures morph into the new ones. This doesn't happen on this occasion. (00:10:00)
Weiner of Our Discontent - S4-E18
Continuity mistake: When Rodger comes to the table, he takes a waffle from the plate, starts eating it and then puts it on the plate. When Steve enters the room and sits down, the plate goes from no waffle to waffle again.
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)
Continuity mistake: Stan's crocodile skin tie changes back to his ordinary black tie when he says "Prison rules" to himself.
Continuity mistake: The poop on the edges of Roger's robe that Stan wears disappears in later shots.
Continuity mistake: We see an old lady shoot a yellow arrow at Roger's golf bag. The shot changes and when we see the arrow again it is red. (00:09:35)
Blood Crieth Unto Heaven - S8-E10
Continuity mistake: When Stan and Francine are first in the kitchen, a record player is briefly seen to the right of the screen. However, when Stan returns to the kitchen after all his dad's items have been put in the bag there is no sign of the record player in the other room. (00:03:15 - 00:03:55)
Continuity mistake: Stan hears Francine's parents at the door, and Klaus is in front of him with a bowl of nachos to the left of Klaus. In a few shots, Klaus's bowl and the nachos switch spots.
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.
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)
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.
Chosen answer: It is just the decision of the creators to do so.
MasterOfAll