Wesley: Xander has just illustrated something. Chances are, you’re all going to be thinking whatever you least want Buffy to hear. It’s a question, of course, of mental discipline.

Giles: He’s right. There are…

Wesley’s Thoughts: Look at Cordelia. No! Don’t look at Cordelia! She’s a student! Oh I am a bad man. I’m a bad, bad man! 

(Buffy stares knowingly at him.)

Wesley: Excuse me. (flees)

Buffy the Vampire Slayer — Season 3: Episode 18 — “Earshot”