While trying to make a walk/run animation CTRL+SHIFT+V doesn't work correctly So, I'm following a tutorial to create a character + his walk and run animations.
The tutorial says to create half an animation and then we copy and mirror the 2nd half.
In the video - everything mirrors correctly for the guy, but for me mirrors the upper half of the body correctly (same position, angles etc. of the arms), but legs get deformed a bit.
As seen in these pictures (frame 1 and frame 9 which needs to be opposite to frame 1)
The symmetry of the IK controllers is bad, here is what we have currently:

If I delete the right leg and symmetrize the left leg, here is what I have:

Now it works correctly, except I'm not sure about your feet Copy Rotation constraint but it's another problem... (I would rather parent the feet to the controllers and give them a Copy Location with the lower legs as targets and the Head/Tail value at 1).