When someone sings a song and does not know the proper lyrics of the original and ad-libs their own lyrics. They usually think the incorrect lyrics they are singing are truly the words of the song.
Todd: "Last nite, Valerie was singing karaoke to Men at Work's "It's Summers Day""
Brent: "Yeah, she kept singing "It's a Mistake, It's a Misssstake. Instead of "It's Summers Day""
Todd: "Dude, that was hystericalyrical!!"
Brent: "Yeah, like when I rambled through Barenaked Ladies "One Week" lyrics!!!!"