During Ben's lip-syncing performance of Roy Orbison's "In Dreams", Frank abruptly ejects the cassette tape midway through the song. A short while later, he has Paul insert the tape in his car tape player. It immediately starts playing at the beginning of the same song without being rewound.
After Frank and Ben received their glasses there's some froth topping Frank's beer, but not Ben's. However, in the close-up of Ben there's clearly a thin layer of froth. Back to both in frame, and again Ben's beer has no froth.
The fact that a seventh-floor apartment exists where In reality there are only six floors Is not a mistake. It Is done purposely and occurs similarly In many movies to deter "sight-seers", fans and psychos from disrupting the lives of people who live In the real location ( For similar reasons, "555-" Is nearly always used on film and TV as the first three digits of 'phone numbers, to prevent people from trying the number and annoying people).
During the first time Sandy and Jeffery hear Dorothy sing, the dress she ends with on stage is different than the dress she started with. However, this could simply be the result of a costume change between the sets of her performance.