Not exactly a joke, but in our last gig (I'm the singer) I introduced our last song this way:
"This is a new original song. We wrote it yesterday and it's only halfways. It doesn't have a title yet, and I'm gonna make up the lyrics right now as I sing. Since we only have half song, we'll play it twice without stopping because if not, it would be too short. And you won't notice."
People moshed like hell, and no one gave a s**t about what I was screaming.
I love being in a metal band.
+1

That's about how I go about punk song writing...