Factual error: When Gary unscrews the top to the nozzle of the front tire of Karen's bike it automatically starts to make a loud hissing sound as if it's losing air rapidly. This wouldn't happen as although the tire would lose air over time it wouldn't make that loud hissing sound unless there was a rip or tear in the rubber.
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.
Revealing mistake: When Rick is having sex with Carmela, it's obvious that it's completely fake. The filmmakers tried to hide it by filming through the keyhole and only showing half their bodies but at one point you can see that Rick is lower than Carmela. Also Carmela is voiced over because her voice doesn't match her mouth.