Frank and Rosie are supposed to be celebrating their 50th anniversary. However, the picture on the wall of their wedding is dated 1948, which is 50 years before the film was released in 1998. However it's set in 1985, which was 37 years since 1948.
At the very beginning of the song "Love Stinks," a loud snare drum hit can be heard but the drummer's hands do not appear to be moving, at least not in the way they should for a loud snare drum hit. In addition, for the remainder of the song, a hi-hat sound can be heard but the drummer's right hand is hitting the snare drum. Drummers typically use their right hand on the hi-hat.
At the end of the double date, when Robbie opens the car door for Julia, no music can be heard until the door opens. However, when the door gets shut, it can be seen that both the driver and passenger windows were halfway down. Music should have been heard the whole time
Robbie tells Linda "Get out of my Van Halen t-shirt before you jinx the band and they break up" That would have been on August 4, 1985 the day before the wedding was supposed to happen. However David Lee Roth had already left Van Halen on April 1, 1985.
During the Cindy and Scott wedding, Robbie includes the Fat Guy in people that will never have anything to offer the opposite sex. When the Fat Guy grabs the microphone during "Love Stinks", you can see a wedding ring on his left hand. This ring appears and disappears in different shots.
There are numerous '80s references (including, but not limited to, the Rubik's Cube, assorted songs, TV shows and fashions) which pre- or post-date 1985 (the year of the movie). While technically anachronistic, it seems reasonable to allow them since the intention is clearly to produce a "period" '80s feel rather than a strictly accurate historical recreation of a specific year.
The meatballs that get squished in Robbie's hands disappear. That's because they are squished; they are no longer balls. The remains can be seen either falling to the floor or sticking to Robbie's hands.
When Julia and Glenn are on the plane to Vegas, she asks him if they could switch seats (referencing a conversation she had with Robbie) because she "had never seen the bright lights of Vegas before." However, since they boarded the plane in the morning/early afternoon, it would have been impossible for her to see the lights in the daytime anyway.
When Glenn gets stopped by Billy Idol, we can see that Billy's large fan is behind Glenn. In the next shot when the fan gets up to defend Billy, the fan is now beside Billy rather than still behind Glenn.
In the aircraft cabin scene near the end, a spiral staircase is seen behind Robbie. The only aircraft with an upper deck is a 747 (four engines), yet the frames of the aircraft in flight are of a narrow bodied aircraft (A320, two engines).