Copy rotation is twisting bone I'm trying to implement a basic foot/leg IK system. I've added the IK constraint and that works fine, and I'm now trying to add a Copy Rotation constraint to my foot so I can control rotation via the IK bone. I'm noticing that when it's in World Space, the foot will twist upward unnaturally.



It works correctly in Local Space, but I'd still like to know why World Space isn't working, and fix it if I can.
It's not working in world space because the target of your copy rotation constraint has a different world space orientation that the foot that is copying that rotation.  Your IK bone is pointed at the ceiling, while your foot bone is pointed at the toe.

If you want to use world space copy rotation, give each bone the exact same orientation.  The easiest way to do this is to make your IK bone a duplicate of the foot bone (but with the parent cleared of course.)