In 1978, Dirk Diggler buys a brand-new Corvette with Goodyear Eagle ST tires. Goodyear introduced the Eagle line of tires in the early 1980s. The ST was never an original equipment tire, it was available only as an after-market purchase.
When the three guys discuss a drug deal with Rahad (set in at least 1984, based on the songs "Sister Christian" and "99 Luftballons" from that year), the audio mix tape runs out the moment the song currently playing reaches its climax, before reverse-side playback. A close-up shows a Maxell 'clear style' tape, not an early-80's style.
When Amber and her ex-husband met with a judge about custody
in September of 1983, portraits of President Ronald Reagan and Governor Jerry Brown are on the wall. Brown was governor from January 1975 to January 1983; George Deukmejian had been governor for several months by September 1983.
When Rahad is explaining that he likes to make mix tapes, he asks Cosmo which one they're listening to right now. They both agree that it's #11. The tape runs out, and when the camera shows a closeup of the tape auto-reversing, it's clearly labeled #6.
(around 1:09:45) During the New Years Eve scene, Todd Parker can be clearly scene walking directly in front of the couple having a discussion at the table, holding a beer and without his jacket socializing like he's been there awhile. Fast forward to 1:12:00, Todd enters the scene and recognizes Reed and they start talking, and it's obvious that he had just arrived, wearing a different shirt and a jacket.
Before entering Rahad's house, Todd places a large handgun in his front waistband. It is clearly visible later as he first approaches Rahad to give him a hug. The gun disappears until he finally brandishes it.
During the pool party scene, the underwater swimming girl gets up. The camera pans as she passes, and the dark reflection of the cameraman holding the underwater camera is visible in the sliding glass door.
When Little Bill complains to the cinematographer about his wife having sex with other men, he says, "My fucking wife's on the driveway with a fucking ass in her cock." Little Bill was supposed to say "My fucking wife's on the driveway with a fucking COCK in her ASS", but the director loved the way the incorrect line showed how angry and flustered Bill was, and left the scene intact.