Factual error: Connor has been raised since he was a few months old by Holtz, a man with a British accent, in a dimension where there are no other humans. There is no possible way he could have an American accent, yet from the moment of his return he speaks with one.
Factual error: Spike suggests that Illyria and Drogyn play Crash Bandicoot. They appear to be playing on an original Xbox console. The episode was broadcast on May 12th 2004, meaning that the only Crash Bandicoot game they could possibly be playing would be The Wrath of Cortex which came out on April 5th 2002 in North America because every other Crash Bandicoot game that came out before that was only on the Sony PlayStation (with the exception of Crash Nitro Cart, but they're not playing that one). Both Illyria and Drogyn appear to be playing the game. However, The Wrath of Cortex was a single-player only game. They're not playing Crash Twinsanity (available for the Xbox) either as that didn't come out in North America until October 1st 2004 and it too was a single-player only game.
Factual error: According to the show, Angel learned quite a few languages when he was evil. However, when he tries to start a conversation with a red-faced demon, he makes terrible mistakes. He said "Hablo Espanol?" which literally means "Do I speak Spanish?" and he said something like "Are you from Byelorussia?" in a tongue that hardly resembled Russian, although according to the captions, it was said in Russian.
Factual error: Wesley has his throat cut, deeply enough to at least nick the artery (based on the amount of blood) and the windpipe (based on his difficulties breathing and talking later on), yet he somehow survives being left alone in the park for over 24 hours. In reality, he would have been dead long before then.
Factual error: When the mysterious person hands Cinco the business card during the 50's flashback (in the restaurant where he is sitting waiting for business), it says "Wolfram & Hart, Attorney's at Law." Clearly, there should be no apostrophe in "Attorneys" in this context - not a mistake a professional law firm would make.