First, let me say that I was inspired to start this topic from a post by Drifkind a few hours ago titled Slip Nut.
This is by no means meant to upstage him but rather to share, for your amusement, something I did about a year ago. It is based on a friends puzzle that I reverse engineered with a slight modification. It is a bolt with 2 ends capped and in between are two nuts. One RH thread nut and the other a LH thread nut. One nut threads to the right and the other threads to the left.
On the bolt, the RH thread is more or less a std design and the LH thread is a coil with fewer threads per inch and is a larger diameter because it rides, on top of, and is subtracted, from the RH thread. The RH nut is std and the LH nut has the coil thread. Confusing? Hopefully my screen grabs will help visually explain how it was constructed. Also, here is my Shapr3D file for those interested.
Bolt-Nut-Puzzle.shapr (1.2 MB)
The 2nd picture shows the separate RH/LH threads, then both combined, then the LH thread subtracted from the RH thread. The corresponding nuts were constructed with sufficient clearance for 3D printing of which I printed a bunch of for my friend to share. Enjoy!