That means trhat your pose (to put it simply) is not tweening in a direction you want it to tween.
One of the remedies is to add one or two intermediate keyframes, to make sure things are going in a right direction.  

Another way is to go inside the advanced animation controls (graph display) and inspect the curves on the offending body parts. You're likely to see the numbers for twist or rotation etc go way out of range. In the graph display, you can drag the curves to the places you want them in, add or delete frames as needed.

Hi, my namez: "NO, Bad Kitteh, NO!"  Whaz yurs?
BadKittehCo Store  BadKittehCo Freebies and product support