By "flip" I assume you mean that the joint is bending backwards?
I haven't done the caveman tute but I assume that it's a joint bending the wrong way while using IK. That wierd flippy-twitchey action sometimes happens when the bones/skelegons are created on a perfectly flat plane.
Couple things you could try. One is to limit the pitch (or whichever) in the motion options for the offending bone. Or you could also try to add some stiffness to the bone.
The best way to fix this though is to pre-bend the joint a bit. Don't have all the points for the bones in a perfect plane. For example, move the joint for the knee toward the kneecap so there is a bend in the joint before you apply the IK.
Hope this helps.
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