Continuity mistake: When Eric and Hyde are speaking while sitting on the couch, Eric's left leg is either over his right leg or under it, depending on the camera angle.
We're Not Gonna Take It - S6-E6
Continuity mistake: When Eric is carrying the tray and they're trying to get him to drop it, he hops over a chair. The tray rotates between cuts. Look at the glasses.
Won't Get Fooled Again - S6-E13
Continuity mistake: When Eric bursts into the kitchen to tell Kitty and Red, who are sitting at the table, that Donna is not pregnant, the items on the table change position depending on the angle of the shots.
Squeezebox - S6-E20
Continuity mistake: Kelso gets really close to Eric when he's trying to burn him about the other girl. The shot changes, and Kelso is further away.
Continuity mistake: After Eric's mishap with Donna's wedding dress, Kelso is in the Forman's driveway with the canoe (yes, a canoe), and the scarf around Kelso's neck changes repeatedly.
Chosen answer: He isn't. Time moves slowly in the show because they didn't want it to turn into "That '80's Show". In the first episode, he say's he's 17 but the second episode was his 17th birthday so he must have been 16. The first 12 episodes are set in 1976, the next 2 seasons are set in 1977, the next 3 seasons are set in 1978, and the remaining shows take place in 1979 with the final show on Dec 31, 1979 when Eric must be 20 as he turned 17 in 1976.
Myridon