To annoy people during the Christmas season:
Sing "Grandma got run over by a reindeer" but at "but as of me and grandpa, we believe" say "that" so you get:
Grandma got run over by a reindeer, walking home from our house Christmas Eve, you may say there's no such thing as Santa, but as of me grandpa we believe THAT...

You get the idea. Extra credit if you get other people to join in your carol. More extra credit if you go caroling and you sing this until the person at the door slams it in your face.