The only time that I've heard of this happening is when a snake mistakes its own tail for food.  This can happen if/when a food item has been rubbed along the tail portion of the body when the snake feeds.  As you illustrate with the photo, the snake pictured has a lump in it already (it has just consumed food) and likely the food had contact with the snake's tail, giving it "food" scent, which is what instigated the strike and consume response in the snake.

Hope that helps!

Rebecca

Thank you for the quick response. Now I have a follow-up question. Why doesn't it stop when it feels pain? Or is it unable to let go?

Well, I've been doing a little investigating, and I have not been able to find a definitive answer for you.  I suspect that the "feeding instinct" is stronger than the "pain" sensation, at least temporarily, but I also know that snakes have backward pointing teeth and so it is a bit of a job to get something out of a snake's mouth once it's in the snake's mouth.  My husband's hand was mistaken for food by a program animal of mine, a bullsnake, and we had to run water over the affected limb and snake's head in order for the snake to "let go".  It did end up in at least one broken tooth on the part of the snake, but no lasting repercussions, other than that my husband would not have anything to do with the snake after that.

Sorry for the delay in the answer - thought I might be able to find you something that was a bit more definitive but not yet.