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).
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.
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 Bar Mitzvah when George is playing the clarinet, both of his lips are sticking out like he's playing a kazoo, including his lower lip, which should be positioned between the reed and his teeth.
Near the end of movie the pilot tells the passengers that they will be traveling at 26,000 feet. Above 30,000 feet altitude aircraft travel at odd number thousands when on a easterly headings. Traveling from Ridgefield, Washington to Las Vegas would be a southeast heading.
When flying to Las Vegas, Julia remarks that she would like to see the lights of Vegas from the plane. However, the plane was a daylight morning flight flying westerly from New Jersey. As the flight would take 4 hours, even if they would leave NJ at 5am and they would arrive at 7am in Vegas time. So there's no way they would arrive at night to see the lights.
Although purportedly set in 1985, there are numerous 1980s references (including, but not limited to, the Rubik's Cube, assorted songs, TV shows and fashions) which could only exist in earlier or later years. While technically anachronistic, it seems reasonable to allow them since the intention is clearly to produce a portrait of overall nostalgia for the decade, 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 Billy Idol first begins speaking over the plane's loudspeaker, he says that one of the First-Class passengers wants to sing to one of the Coach passengers. Julia instantly appears to think its to her, but she has no reason to think this. Also, when Robbie begins to sing to her still in First Class, she turns to see the entire cabin of people gazing at her with smiles. The people had absolutely no way of knowing it was her that Robbie was singing to.
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.