Just prior to being egged by Lindsay, yellow yolk can briefly be seen on the right side of Sam's face as he is arguing with Neal. The shot immediately before and after, however, Sam's face is clean.
When Sam is arguing with Neal just before Sam get egged, in one of the frames Sam has the egg makeup on.
Lindsay Weir tells her friends, "We could go to the movies. There's a new Friday the 13th playing." By saying new she is implying it is a new film (a sequel). Friday the 13th came out in May 1980, the scene takes place in October 1980. A "new" Friday the 13th (Friday the 13th Part 2) was not released until May 1981.
Millie calls the candy she is eating at the bus stop Lik-M-Aid even though it says Fun Dip on the package. Lik-M-Aid was the original name but it was changed to Fun Dip in 1989. The name Lik-M-Aid is on the current Fun Dip package but in smaller letters above the its current name of Fun Dip.
While the episode is supposed to take place in 1980, Daniel drives a 1983 Cadillac Sedan DeVille. In addition, 1990s cars can be seen in the background as he and the Freaks are riding around.
Harold's mouth is closed throughout his line "Who the hell did this to you, Sam?"
Halloween takes place on a sunny summer-like day with green trees in the background. In southern Michigan on 31 October it should look like autumn, and since it gets toward dusk around 6pm at that time, the action would have to take place in mid-afternoon.
In the opening scene of "Tricks and Treats" when Sam and Neal are making a gross smoothie for Bill, an open jar of peanut butter is sitting on the table as a possible ingredient. It is revealed in "Chokin' and Tokin' that Bill is highly allergic to peanuts.