If you're not familiar with the Chili Peppers, their drummer Chad Smith and actor Will Ferrell look exactly alike. Their striking similarity has lead to a bit of a public feud between the two. So Jimmy Fallon had them on The Tonight Show to try to quash that and compete in a drum-off. 
Check out Chad and Will's skills (which were all provided by ?uestlove while Will was air drumming) and the unexpected ending.