I was watching "Angel" (the ep, not the series), and when Angel and Buffy are chased back to her house by the three, the fact about vamps not being able to enter without invitations is first given.  Besides noticing that one of the three still manages to get a hand inside before Buffy slams the door (oops!), when is the first time we actually see a vampire bounce off a barrier?

If I'm not mistaken, "Passion."  Or the one where Xander does the love spell, maybe, cause I know Drusilla tries to get in, and Angel makes fun of her.  On second thought, I do think  it is the love spell ep.