McCoy saying "he's dead, Jim" over the horned doggy thing in The Enemy Within. For that matter, the horned doggy thing.
McCoy suddenly needing to get back to the Enterprise as soon as he's cured in For tthe World is Hollow and I have Touched the Sky. Natira, I love you, but I've got to go now, I have so much noble stuff to do, yeah, that's it...
This didn't strike me as funny until I started chewing it over, but say this in your head a few times, from This Side of Paradise:
Spock: I am what I am, Leila, and if there are self-made purgatories, then we all have to live in them. Mine can be no worse than someone else's.
What the hell does that mean, exactly? Sounds dramatic and highfalutin' as all get-out, but, er, it means...? We have to all live in any self-made purgatories we have constructed? This is common, is it? And why can't Spock's be worse than anyone else's?
Now when I hear this, I crack up. You go writers!