A good friend of mine has a nasty nack for finding just the right word to replace to totally change the song. For example;

Blue Ridge Mounatin Blues

When I was young and in my prime I used to do it all the time. Now that I am old and gray I onkly do it once a day

But his all-timer came about three in the morning jamming at a festival. A friend was with us, determined to stay up all night because they were to be on stage early next morning. He mentioned he'd be singing "I Wonder Where You Are Tonight". Charlie (with the nasty nack) says, "let's sing it" and when the title line comes up he sings "I'll wear your underwear tonight."

Rocky gets on stage the next morning and all those who helped him "rehearse" his song in the wee hours were in the front row to cheer him on.

Sure enough, he gets to the chorus and sings

"And though your love is even colder, I'll wear your underwear tonight."

If the audience caught it, they were very polite (unlike those in the front row, who were probably like the 8 year olds during the "snot" in the Lord's prayer, without the mother's stern look.)