I've got one:
Bentley had a really modest nub of a tail to begin with when he was an older puppy but it slowly appeared to decrease in size over the course of a few months. So I got online and started googling away different causes of the apparent shortening of a tail. Within minutes, I had myself convinced Bentley (though completely healthy and happy) suffered from a debilitating spinal condition that was causing his tail to shrink. I made an appointment at the vet to let them know what I was thinking and they look a look at Bentley and let me know all my worry was for nothing - Bentley was just getting fat so his tail was basically looking smaller and less prominent because his butt was getting fat. Boy I felt stupid. :p