Blooper

At the end of Becoming part 1 and the start of part 2, Kendra has had her throat slit by Drusilla and drops to the floor, presumably dying of blood loss. When Buffy discovers her body, there is no blood to be seen at all. Although this may simply be for ratings / censor purposes.

As has been established, vampires do not have breath. However, Spike is often seen smoking which requires drawing smoke into the lungs to even light the cigarette.

Near the end when Spike gets Drusilla in a 'sleeper hold', she passes out. It has been established in the series, and vampire mythology in general, that vampires don't have a pulse and don't breathe. Therefore, a 'sleeper hold' would do nothing but restrain a vampire. They would not pass out.

When Spike is carrying the "unconscious" Drusilla, you can see her hand grabbing on to his shoulder.

Obvious use of doubles during sword fight.

During the sword fight, Buffy climbs to a sofa and jumps down again. As she does so, Angelus is seen to be slowing down his blow, in order to allow Buffy to land properly and continue with the 'choreographed' swordplay.

During the sword fight between Buffy and Angel, the stunt doubles can be easily identified, by Buffy's double having more brown like hair and Angel's double having a significantly different hairline.

It's a minor thing, but during the sword fight with Angel, both Buffy and Angel repeatedly have their forefinger over the crossbar of their swords. That does give better control, but no one with any sword skill would do it with that style of sword. It is assumed Buffy has had sword training, some, from Giles, and Angel's old enough to have done some.

The problem with the forefinger over the hilt is swords slide, and that's a great way to lose a finger. A person can do that style with a rapier, because it has a guard beyond the crossbar. Never with a straight hilt sword.

It is known by now vampires have no breath. Around the 13:20 mark you see Angel breathe onto Giles glasses and clean them. It's nothing more than a scene filler and doesn't harm the show at all.

Since Spike is betraying Angel, it makes no sense for him to start whacking him from behind with what seems to be some sort of tire iron. He could just as easily have staked him for good and be done with it.

The Acathla ritual entails that Angelus pulls the sword out with his own blood on his hand causing Acathla to swallow the world into Hell. Only Angelus's blood will close the portal. Logically only the mansion and the surrounding area would get sent to hell before Angelus's blood would close the portal. Given how secluded the mansion was, the damage would have been fairly minimal with no casualties except for Giles. This is assuming Buffy and Xander never bothered to go. In other words the episode wasn't nearly as urgent as it was making itself out to be. This was just about saving Giles.