Kumar: I once took a Ko-re-an guy's toothbrush, and rubbed it all over my dick.
Klan Member: Now that's just disgusting. But I like it!
"Dude, the klan is fucking slow."
When the old woman sees Kumar sitting some seats ahead of her, she briefly imagines him with a beard and turban, laughing at her, and imitating an airplane with his hand, falling out of the sky and making an explosion sound.
The scene where Neil Patrick Harris does mushrooms and has a vision of himself on a unicorn. Even better, he's being questioned by a police officer the entire time. Not once does the cop even suspect that anything is wrong.
The racist (and apparently anti-semitic) Agent Fox tries to break Rosenberg and Goldstein during interrogation by slowly pouring a bag of pennies out on the table in front of them. What makes it more hilarious is Rosenberg's actor can barely contain himself throughout all the takes.
At the brothel, Harold is telling all the prostitutes about how much Kumar has been screwing up the whole journey through and is probably thinking that he's having a great time having sex with the prostitutes. Cut to Kumar's room and we find him crying over losing his girlfriend while the prostitutes try to comfort him over his loss.