Deliberate mistake: In the flashback to Angelus rising from his grave, his and Darla's breath is visible as they exhale. This was going to be removed in post-production, but left in due to budget restrictions.
Cold Stones - S6-E11
Deliberate mistake: Tony smashes AJ's front windshield of his Nissan SUV one time with a plastic sports helmet and it leaves a hole 4 times the size. Not only is the scale size of helmet to hole incorrect, but front vehicle windshields don't shatter like that. Modern windshields are made of laminated safety glass, which consists of two curved sheets of glass with a plastic layer laminated between them for safety, and bonded into the window frame. It would've taken many impacts to cause such a hole, not one hit. (00:49:00 - 00:49:45)