As anyone who's every attended an elementary school concert knows, there are ways that songs can be butchered which are beyond our ken. I'll give a couple of examples here.
Jimmy Kimmel and Mike Tyson duetting "Winter Wonderland" is one of those where you stare in total incomprehension. I suspect they picked this particular song so we could here Mike lisp his way through the "snow is glistenin'" line. The Casio is the only possible background music for this.
"O Holy Night" is one of my favorite Christmas songs, but it's a challenging one to sing. This guy accepts the challenge and fails spectacularly. It starts off seeming like just your basic bad cover, but give it time.