Following 'The Essential Blender' and ran into a problem...

So I picked up a copy of ‘The Essential Blender’ and have been following the rigging chapter to the letter and I’ve run into a small problem that isn’t addressed in the book. The section where he talks about adding a knee to help control the rotation of the lower leg in relation to the foot. Everything works fine until I add the knee bone and move/scale it into place. I have a couple screenshots to better describe what is happening.

In edit mode, I select the lower leg, shift+d, move the new bone forward a little, and LMB to accept. Since I have x-axis mirror on, a bone is created on both sides and everything looks good so far. HOWEVER, when I tab back into pose mode, the new left bone’s tip jumps back to the ankle joint, bending the leg in a weird direction. The right (mirrored) side, however, looks and behaves normally!

Can anyone suggest what I’m doing wrong? (I realize I’m using v.2.49b and not the 2.44 included in the book, but I just installed blender from the apt repository.

After creation and renaming of the new ‘knee’ bones.
After switching back to pose mode.

Are you using an IK on the bone? It’s been a while since I’ve used 2.49, but if I remember right, constraints aren’t mirrored. It sounds like when you duplicated the bone, it also duplicated the IK Solver constraint on the bone, but not on the X Mirror. So, to solve the problem, just delete the IK Solver constraint (Go into Pose Mode for the armature, select the bone, go to the Editing tab, the Constraints panel is on the left.)

Looks like that was my problem! Thanks, Holster!

Yeah, in the book he has you add the IK solver and play around with the foot, but makes no mention of removing the solver before creating the knee. I wonder if this is an editorial error or a change in the newer version… Oh well.