Glinda informing Oz that he will go through the bubble wall, if he is good at heart. Oz quickly realizes what she just said, and adopts an Oh Crap! expression. This is followed by him yelling, "I'm gonna die!"
Even funnier, that's the second time he says that. Glinda's expanation of the bubble wall was supposed to be reassuring. It wasn't.
Oscar repeatedly failing to remember everyone's name properly.
Theodora making Oz "dance" is actually darkly funny. Until she slams him into the wall.
Also, Evanora's unnerved looks whenever her sister goes Ax-Crazy and starts hamming it up.
Almost everything that Finley does.
Actually, the entire movie is incredibly funny, because almost every scene somehow has Narm Charm.
Example: "Begone, Witch! Before the Wizard makes mush of you!"
Evanora fleeing to the inside of the castle after getting the crap scared out of her by Oscar's "immortal form."
Before they go into the Dark Forest two crows caw at them before saying this:
Crow: You'll die! You'll die! You'll die!
Oscar: (unerved) Did those crows just say we're gonna die?
Finley: (scared) They did...
This is probably a more subjective example, but if you're not scared by the plants in The Dark Forest and you think the way that Oscar, Finley and China Girl completely freak out when one drops down and suddenly appears in front of them is just a bit of an overreaction, it's actually quite hilarious. The highlight is when we see them from the plant's point of view complete with yellow vision and zoom-ins on their terrified faces, all while Oscar is screaming "AAAAAAHHHHH, WHAT IS IIIIIIIIT?!!"
The evil flying monkeys tear into what they think are enemy soldiers... only to find straw and hay in their hands instead of flesh and blood. One of them gives the Wicked Witches a "wtf?" look.