When we first see the grown-up Frank outside the cabin, he is missing his shield on his shirt. Then it magically appears inside the cabin.
Cadillac kept switching between 1969 (exposed taillight lenses) and 1970 (hidden taillight lenses). Plus the cars alternated between no passenger door mirror, Cadillac passenger door mirror, and aftermarket passenger door mirror.
Movie happens in real time 1998. Twenty two years earlier would be 1976. Chief O'Brian says why would you have comprehensive insurance on a 10 year old car. It was only 6 or 7 years old (because both a 1969 and 1970 Cadillac was used).
It is said you can't get in the trunk without the key. The Cadillac has an interior trunk release.
When the boys push the Nova off the cliff into the quarry, the hood and trunk covers fly off, revealing gaping holes where the engine and drive shaft should be. They have obviously pushed an empty shell off the cliff.
The phone booth in the early flashback scene clearly wobbles when touched and is obviously a lightweight prop.
When the young George is surrounded by the bullies outside the liquor store and the other three kids walk up, the boom mic can be seen in the lower foreground for several seconds.