Kat, thanks for the link to that holiday mondegreens website.

My favorite is "Noel, noel, noel, noel, Barney's the king of Israel." LOL

Of course, "Let men their sins enjoy" (Joy To The World) and "Six geezers laying" (12 Days Of Christmas) are close runners-up.

Another Christmas song that's often "mondegreened" is "Christmas Time's A-Comin'."    The line "I can't walk for runnin' - which tends to be pronounced "[Ah] cain't walk fer runnin'" - has been misheard as
"Cane foam's a-runnin'"
or
"Hockey foam's a-runnin,'"
and you'll find those posted fairly often online as the actual lyrics.