I honestly did see this! Today I was walking down the bike path trying to race a thunderstorm home when I saw a black snake out of the corner of my eye. It seemed to be crawling slower than usual. I stopped and noticed that the snake's tail appeared to be up in the air while it was crawling away very slowly.
Looking closer, I realized the end that was up in the air was the head. It had something in its mouth and it was slowly crawling BACKWARDS away from me. Suddenly it swung its head around and shot off (forwards this time!) into the grass at the typical lightning speed you would expect from a black snake.
A cursory search online indicates that snakes are not supposed to be able to crawl backwards. Well, maybe they are not supposed to be able to crawl backwards, but nobody told that particular snake. Granted, he wasn't very good at it, but he was definitely headed in a rear-ward direction.
Personally, I would prefer not to see snakes on my walks at all. If I do see them, however, I do like to see them headed in the opposite direction ... forwards or backwards.