Revealing mistake: When Stan is at the launderette, a man is shown writing on paper. There is no ink shown on the paper. He's also only writing on one single line and not moving down after finishing the line. (00:01:10)
Continuity mistake: The way the glass is shattered keeps changing between shots when Bullock jumps out of the diner window. (00:19:35)
Factual error: Steve and Roger make a prank phone call to Tony Blair at 4:30 AM GMT during sunset in Virginia (EST/EDT); because of the 5-hour time difference (4 hours during Daylight Savings), it would be 11:30 PM or 12:30 AM in Virginia, which would be nighttime.
Continuity mistake: After Steve accidentally jabs the wood burner into Stan's penis, he screams and a avalanche occurs. Then when Stan screams a second time due to putting the cream on his penis, the avalanche happens again but the same amount of snow is on the mountain even after a load already fell off it. (00:06:40 - 00:07:30)
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: 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.
Revealing mistake: When Stan comes home and starts talking to Francine, as Francine is saying "You're just having a mid-life crisis", there are three objects in the background that are cut in half and seem to be floating in the air. (00:02:10)
Revealing mistake: While Hayley is throwing up into the bin, you can hear the sound of it going into the bin. However, you can see nothing is actually going into the bin while she is doing it.
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)
Failure Is Not a Factory-installed Option - S2-E3
Audio problem: Stan looks out the window whilst being suspended from work. As he looks he sees two people exiting their homes and they both say "Bye buddy" and "See ya pal." Their mouths don't move.
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.
Tearjerker - S3-E10
Character mistake: When Tearjerker is describing his evil plot to Stan, he says he auditioned for a role in Monster's Ball in 2002. Monster's Ball was released in 2001.
Character mistake: At one point in the show, Roger throws a bucket of water (in disguise as Scotch Bingeington) on Francine claiming "Wet T-Shirt Contest!" Francine looks disgusted when Roger states "Eh, give this one a four" to the guy judging the contest, then says to Francine "Don't feel bad, you've had three kids." Francine replies "I've had two." and Roger gasps and puts a consoling hand on Francine's shoulder. This is wrong because Francine has birthed four children. She previously gave birth to Bailey (Hayley's twin), and Greg and Terry's baby in Season 3 Episode 7 "Surro-Gate." (00:14:15)
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.
Chosen answer: It is just the decision of the creators to do so.
MasterOfAll