When Kelso first drives up to the Foto Hut, there is no smoke inside the room. However, when seen from Kelso's point of view, the room is filled with smoke.
Red puts salad oil in the water to prevent it from boiling over. The salad oil is not in the water long enough to make a difference.
The first time Donna sneaks into Eric's bedroom, he has Spiderman bed sheets. The Spiderman logo design is from the Spiderman animated series that didn't appear until the early '90s.
When Red serves Kitty the Kentucky Fried Chicken (trying to pass it off as home cooked), the meal includes a biscuit. In the '70s, KFC served soft dinner rolls, not biscuits. Biscuits were not added to KFC meals until the early '80s.
In the corner of Eric's room, there is a "Cheesehead" hat. "Cheeseheads" were not invented until 1987.