On a par with forgetting the first verse when you repeat it as the last verse is:
repeating the chorus at the end and getting it wrong the second time; and
carefully reminding yourself of all the verses before you start but falling into a black hole when you reach the first chorus....

and I have frequently improvised - if you just keep going, people don't always notice that you've changed the words, especially if you can still find a rhyming one at the end of the line!

Kitty