Deliberate mistake: While it is established that Angel doesn't age, it's obvious across both Buffy and Angel that David Boreanaz gets older. More unavoidable than deliberate, but still worth noting.
Deliberate mistake: Though vampires are said to possess super-human strength, and are in fact demonstrated to have it, they are often kept confined by the flimsiest of restraints. One distinct example is in season 2 episode 9 when Kendra locks up Angel he is unable to break out despite being held in by nothing but a flimsy padlock. This mistake is pointed out in the DVD commentary as an unavoidable 'mistake'.
Character mistake: Time and time again, Buffy or another of the Scoobies have gone a few metres away from a vampire and talked about them, shouldn't they be able to hear them? Because aren't they supposed to have heightened senses?
Other mistake: In episode "Helpless" in season 3, after the council member bitten by the crazy vampire rises, his bite marks heal. But in season 7's "Lies My Parents Told Me," it shows Spike's mother already risen and her bite marks are still there.
Chosen answer: "So goes the nation" seems to have been used on many occasions, with various different US states in the "As .... goes" section. Most commonly it seems to be California that's considered to lead the way, but probably most other states have appeared in the lead role at some point or another. Other things have also been used - no less a person that Pope John Paul II said "As the family goes, so goes the nation...". The origin of the quote format is unclear - in US politics it goes back into the 19th century, when it was Maine that held the title spot, but, while no definitive origin is known, it seems highly likely that it goes back considerably further than that.