If Gwen's room is in fact Donna's old room, then it means it somehow moved from the second floor of the house to the first.
When Nate tries to open the keg without a tap, Gwen's nose ring is visibly missing.
When the kids are on the water tower, you can see Jackie's name on the tower, but her name was covered up by Fez in the final season of That 70s Show.
Donna states that Eric is 38, but he turned 17 in 1976 so in turn that would make him 36 in 1995. This, however, is not considered a Goof per IMDB guidelines. The below is one of the items for "Unacceptable Goofs" for long-running TV shows (NB: the last sentence):
These get a fair amount of latitude when it comes to continuity between episodes. It seems unreasonable to us to expect that every detail of every character's life, every setting and situation, every "fact" in the imaginary world, will be fully thought-out and properly documented when a TV series is first devised. Teams of writers work on these things over many years and it is inevitable that newer, better ideas will come along that will supplant older notions. These will not generally be considered as continuity goofs (unless they're especially interesting or amusing).
Gwenn is seen to jerk her CD player sharply a few times while dancing. This would have caused the player to skip or error out.
After Kitty greets Eric by, as he puts it, motor-boating him, Topher Grace spends the rest of the scene with a visible spot from Debra Jo Rupp's makeup on his shirt.
When Leia tries to buy a tap for their leg at Mama's, the bottles of liquor on the shelf are current releases, not the ones that would've been available in the 90s.
Around the 13:24 mark on the left at the top.
(at around 13 mins) The end of a microphone can be seen to move in and out of the top left corner of the frame.
Donna Forman (Laura Prepon) says that Eric (Topher Grace) is 38 years old. However, in the original series, the gang graduates in 1978 which means that Eric is either 35 in 1995 or he was 21 when he graduated high school.