Joe the KingGoofs
Continuity
Joe licks the envelope closed to the card he leaves his mother under the porch, but when she crawls under the porch to open it, it is unsealed.
Revealing mistakes
Toward the end of the movie when Joe's father is driving to pick him up, he passes a driveway with a Jeep Wrangler parked in it. If it were the 1970s, it would have been a Jeep CJ-5 or CJ-7.
Anachronisms
Although the film is supposed to take place during the 1970s, Joe is seen wearing a name brand underwear with the company logo on the waistband. This particular brand didn't produce that style until the late 1990s.