More of a cautionary tale than a question...
I posted this question over my lunch hour earlier. (Briefly, generate some type 4 GUIDs using the cryptographic random number generator.)
I wanted to avoid "easy answers", so I came up with some rules. One of those easy answers was to grab 16 bytes of randomness and to drop in the six fixed bits, so I added the rule that random bits can't be wasted.
That rule would require that a fraction of a byte would need to be loaded from the random number generator (too hard) so I specified that the solution should output four GUIDs. That would allow the program to load a whole number of bytes and use some bit-fiddling to arrange them into four GUIDs.
So, with my "perfect" challenge in place, I posted it with my rules. No problem?
Trouble was, my rules probably seemed a bit too arbitrary and so were ignored. I had to later edit the challenge to explain why I made these rules. Only one of three answers posted as I type this actually conform to them.
tl/dr: Don't assume people will understand why you've put a rule in place.