When Kelso is lying in bed before he has the I Dream of Jeannie (1965) dream, his green phone is a rotary-dial phone. However, when he wakes up and makes a call, he dials the phone as if it was a touch-tone phone and DMTF touch-tones are heard.
There's a can of Pringles in the mini-fridge. It has the modern-day design instead of the design used in the '70s.
When Mrs. Forman gives Hyde and Fez Snack Pack Pudding, it is in a plastic container with a foil tear-off lid. In the '70s, Snack Pack Pudding was in a cardboard container with a tin bottom and a tin pull-top lid. The plastic container didn't come out until 1984.
Red asks Eric if Kelso's parents are as dumb as Kelso. In "Career Day" (s1e18), Kelso's father works as a "Senior Executive Statistical Analysis Technician", which could be considered an intelligent profession.
When Red asks Eric what the square root of "X" is, Eric replies, "'X' is a variable, so until you define its parameters, the only possible answer is a variable, or 'X', if you prefer". While the first half of that statement is correct, the only time that the square root of "X" equals "X" is when "X" equals 0 or 1. Eric should have given another variable (such as "Y") as the answer to the square root of "X" because there is no indication that "X" is either 0 or 1.