Kelso pours ice cubes into Fez's pants. Later, during Kitty's visit, three perfectly-formed ice cubes fall from his pant legs. Obviously, these are plastic cubes, as real ice certainly would have melted by this time.
At 21:50, when Eric is in the basement practicing bouncing quarters and Hyde walks in, you can see what appears to be a large cockroach run across the floor from left to right in the bottom left corner of the screen.
While in the car, after Donna cheers up Jackie, Jackie takes off her hat and tosses it to the side, accidentally hitting Donna in the face.
This episode is set during Christmas, 1977, but the license plate tag on the Vista Cruiser shows '77 for more than 10 episodes afterward.