Akatsuki making a deadpan but completely serious statement, after having apparently stumbled across an unconscious Yuuki in the middle of nowhere.
Hanabusa: What's the matter, Akatsuki?
Akatsuki: No matter how you look at it, Yuuki Cross is lying here.
In the episode Night of vampires, the character says "Their gaze will enslave you" and I Heard That As "their gays will enslave you".
Whenever Yuuki and Zero have one of their banters in the first arc.
In an early chapter, Yuuki and Zero are held back from going to their guardian duties by a teacher, resulting in the day class girls having free access to swarm the night class guys (vampires, unknown to them). Aidou's embarrassed expression upon being surrounded by squeeling fangirls is especially funny.
Kaito asking Yori if she's Zero's girlfriend.
A late chapter revealing that the nurse Zero had a crush on as a child (who turned out to be an ex-human who eventually fell to level E) was engaged to Yagari. Which means Yuuki isn't the first girl Zero's liked that were someone else's fiancee.
Zero's reaction when Yuuki straddles him on the train in chapter 87 to offer her blood. Despite the serious situation, a lot of readers found themselves cracking up at his utterly shocked face.
Any time Kaien acts like the incredibly dorky father he is, especially when he wants Yuuki to call him father.