The cell phone holder for my car broke.  And it broke in a very particular way.  The threaded ring on the back that screws onto a set of tapered fingers that clamp onto a plastic sphere, broke.  In the picture on he left you can see the tink crack at the 6 o'clock position.  

I thought about 3D modeling the part along with the threads to try to repair it.  Then it occured to me that I could fix it equally well by putting a ring around the part. The repair works because it does not allow the inner ring to expand as it tightens. The part prints in just a few minutes and I didn't need to spend hours tweaking the thread and re-printing and disposing of prototype parts.