This is an old story, but it came to mind when I was thinking about unintended consequences earlier. My wife got these awesome kids' bath towels with an animal-head-style hood. Here's Greg wearing the shark towel:
We initially only had three of them. The problem arose that if we had just used two of them and put them in the laundry, then there was only ONE of these special towels left and TWO little boys who wanted to use it.
I thought I was so smart in the way I convinced our oldest (the "rational" one) to use a plain towel and let his younger brother use the animal towel. I told him the plain towel didn't have an animal on it because it was a shape-shifter towel (see also Calvin & Hobbes' transmogrifier). He asked what a shape-shifter was, and I explained to him that a shape-shifter was a magical creature that could change forms from one animal to another, so with THIS towel he could be ANY animal he wanted and he could CHANGE what type of animal he was WHENEVER he wanted.
This was a big hit with Brian...
Unfortunately, it was also immediately a big hit with Greg, too, and now neither little boy wanted the previously-coveted animal towel. They both wanted shape-shifter towels so they could be whatever animals they wanted. It turned into a sort of elaborate game of ro-sham-bo. "I'm a snake! sssssss!" "Oh yeah? Well I'm a mongoose and I'm gonna eat you up!" "Oh yeah? Well I'm an eagle and I'm gonna eat YOU up!" "Oh yeah? Well, I'm a..."
...Not exactly what I intended to happen.
P.S. Luckily, this effect was short-lived. Greg only wanted the plain towel because he saw his big brother wanted the plain towel (monkey-see-monkey-do). In subsequent bath nights, Greg reverted back to wanting the animal towels, and Brian continued to want the shape-shifter towels, so everything was good. Amy eventually found us some more animal towels, too. :-)