This is a little old (August 2009), but new to us: Here's a video of some guy doing a totally insane impression of Sesame Street's Elmo while ordering at a Taco Bell drive-thru. Who knew Elmo saying "chalupa" could be so funny? Or better yet, when he announces in the third person, "And that will conclude Elmo's order."
Video: Extremley funny elmo orders taco bell
Seriously, if you need to prank someone at the drive-thru, it's probably better to do funny voices than throw molten creamed spinach back through the window.