To me, this is the biggest and most common mistake students make while performing Naihanchi Shodan:
The whole idea of Naihanchi is to have a firm base. The stance must be strong and rooted. The stance is destroyed when the leg buckles. To prevent this, the student must learn to press the leg out (or spread the hips) during the elbow strike.
The legs are not supposed to buckle during any part of the kata. I know this to be true because I did it wrong for many, many years. Now I am more careful about this. It is not a hard problem to overcome -- it just requires attention.
If your leg buckles, someone could stomp or kick it.
Charles C. Goodin