Here's an unusual and amazing feat of memorization by OneManSho! At first, it simply sounds like he's singing an opera in Latin or Italian. Even if you're not into opera, keep watching it! About the middle, you'll realize how familiar the song is, and how impressive a memorization feat this truly is!
OneManSho also performed this same feat with another song, but there are stronger visual clues in this video as to which song he's singing.
This idea has been done before, such as in Weird Al's Amish Paradise video (watch from 2:51 to 3:09), as well as the classic Swedish bookstore scene from Top Secret! (here it is the original way, if you're curious). However, OneManSho is doing it while making sense and without lip-synching!