Deliberate mistake: When Elliott and ET are on the bike heading to the forest on Halloween night, ET is covered with the white sheet, but in the closeup of ET's face right before the bike flies he's covered with the white blanket (note the material), which is from the scene later in the movie.
Deliberate mistake: When the girls are done snorting the "coke" and Rick takes the tray away, none of the girls have snorted any of it because the same amount is still there except instead of lines it is now all muddled together. Also if you watch closely when they are snorting the coke, the rolled up bill only gets close to their nostril and not partially inside it. (00:09:45)