Glass

It wasn't this one, but it took me a quite some time to discover that the cause of the apparent failure of my dishwasher was what appeared to be a small piece of broken glass stem.  It had lodged itself under the spray arm preventing the arm from turning.  I have no idea how it got there as I have no recollection of any glasses breaking in there.  One of the mysteries of life I shall never have an answer for. As soon as I found it though the odd noises stopped and the machine sounded much healthier and lo and behold, the dishes actually came out clean.