To follow on from Barbara's example, I have changed the words to songs deliberately while performing, just for the sake of mischief and fun - often to include other members of the band, or some topical event. Often this was for a Standard that I had sung scores of times previously and could sing in my sleep. The trouble is (with my awful memory0 that the mischief becomes instantly ingrained. Trying to NOT do it I now find very hard.
As an example I used to sing a fun Aussie song called Euabolong Ball, with lyrics that went in part:
"Though the water was scarce there was whiskey to spare, what they didn't swallow they rubbed in their (substitute Barbara's or Joe's or whoever's) hair." That works fine until you spontaneously decide to change it to ... "Though the water was scarce, there was scotch in the hall"... and you then have to quickly decide whether to make it rhyme in the next line.