The ones I can't sing with a straight face are old gospel songs re-worked by the likes of Joe Hill. For example
"Hallelujah, Thine's The Glory" became "Hallelujah, I'm a Bum" and
"In The Sweet Bye and Bye" became "There'll Be Pie in the Sky."
On the Sunday before Labor Day, I was asked to provide instrumental music for church. I couldn't resist playing "Pie in the Sky." I don't know how many folks got the joke.

Jim