Kelso says that he's the only one of the guys who's never seen Donna's breasts. However, in "Hyde Moves In" (season 1), everyone went skinny dipping and had seen each other naked.
Kelso and Jackie remember when they were caught kissing in the basement in 1968. A smoke detector is installed next to the basement door. Red hung the detector when he discovered the guys were smoking down there, so it would not have been there in 1968.
Eric and Donna talk about the first time they met, at the age of nine. But in previous episodes, they talk about knowing each other when they were even younger, such as when Donna says, "We've lived next to each other forever. You could have had me when I was four."
Kelso & Jackie got caught kissing in Eric's basement in 1968. In the same episode, they recall meeting Jackie when they were in middle school. Donna lived next door to Eric. They all should have met around the same time in 1968.
When the guys are remembering the first time they met Fez, he was hanging on the inside of a door. You can clearly see a strap with a loop protruding from Fez's shirt and hanging on the hook but when Hyde lifts him to set him down, the camera angle changes and the strap disappears.
The gang is reminiscing about how they all first met. The setting is 1968, and Hyde and Eric are in the bathtub. Eric hands Hyde a rubber duck. Hyde knocks the duck from Eric's hands and a blue circle is visible on the bottom of the duck. These are the ducks that say "HOT" when the water is too hot for babies. These ducks were not around in 1968.