Sorry, Kristen, but it's "when the banjo lands squarely on the accordion in the dumpster without hitting the sides".

Another old one is...

I went down to the mall the other day and just after walking in the main door, I suddenly remembered that my back door was left unlocked and my banjo was on the back seat. I turned right around and ran back to my car, but it was already too late. There were now three more banjos left there.