Ugh, Xander makes me mad sometimes.
Whenever Angel is mentioned, I don't like Xander. Ok, ok, we get it, at first he didn't like Angel because he liked Buffy so he clung to the "zomg he's a vampire!" thing and ran with it....but there's a point where he should have let that go. And ok, ok, we get it, Angelus = bad guy...even so, the "kick his ass" scene (and attitude) made me so mad. And the jumping-to-conclusions thing with Faith in S3. And his total hypocrisy when Buffy went off to kill Anya.
He's also annoying (although, granted, less so) when Spike's mentioned. Yeah, we get it, Spike's evil. But you'd think after fighting alongside the Scoobies, and his obvious love for Buffy, and his blatant desire to make amends, that'd count for something. I detest his reaction when he discovers Buffy's relationship with Spike. Yeah, ok, he has a right to be upset/confused, but he handled it so badly (as I knew he would when he found out, and so did Buffy....why does he think she hid it from him so carefully??)
Another moment of "OH JESUS SHUT UP XANDER!" that sticks in my mind is when he and Dawn were talking, and he blurted out, "Yeah, Spike tried to rape your sister" (or something along those lines..I know he worded it way worse in the episode). If anyone were to explain the situation to Dawn, it should have been Buffy, not Xander. And he said it so insensitively, in such a cold and cruel way, I couldn't believe it. It was as if he just wanted to make Spike the bad guy, not caring how that information would affect and hurt Dawn.
There are probably more. I have very much a love/hate relationship with Xander.