Don't hyper-extend your joints

You’ll see this model-posing mistake more often that you can imagine.  Like other posing mistakes, sometimes you can get away with it and sometimes you can’t.   So be careful when you lock those joints.  Try to keep a little bend in them or at least try to keep them straight.  God didn’t mean for knees to bend backwards and people notice (negatively) when they do.  The problem is most often noticed when a model is standing at a profile to the camera and with the elbow when, the model is in a sitting position and the model is leaning on one arm.

Some bends you can get away with...

And some you just can't