Weighing with One-Foot Scale I stand with left foot on a scale and the other on a piece of effectively incompressible material at the same initial height at the scale's top surface. The weight shown is WL. I repeat with right foot, producing weight WR. i then stand with both feet on the scale producing final reading of WB. Question: is WB = WL + WR ?
 A: If you were to weigh your body one side at a time (left, then right, as you described in the question) it's very possible that you would be off by a noticeable amount.  This is because you have likely shifted your weight around in the process of standing on the scale with your left foot, moving around, standing on the scale with your right foot, etc.  If you have ever stood on a scale with a single foot, you can make the scale reading change pretty easily by shifting your weight.  If you lean more on your left foot, the scale reading will go up, but if you lean more on your right foot, the scale reading will go down.  Since it may not be immediately obvious which way to stand is most natural or most even, you likely will not be standing exactly the same in both trials.  There is a better way that addresses the same concept, however:
If you have two bathroom scales (let's say they're identical, just to keep our relative uncertainty consistent) and you put a scale under each foot, the sum of the two weights will equal your total body weight no matter how you shift your weight.  Since your body is only being supported by the left and right scales, the sum of the scale readings will equal your body weight.  You can shift your body weight around to change individual scale readings, but a decrease of the reading on the left will be accompanied by a corresponding increase on the right, and vice versa.
A: In a theoretical case, this should work.  In real world situations, there are a couple of reasons you could run into difficulty.
*) Non rigid bodies and shift of mass distribution.
If you've got a car or a statue, you'd be pretty close.  But a person or a rhino wouldn't work as well.  As the body moves, the proportion of weight on one end or another shifts, sometimes dramatically.  
*) Multiple contact points.
If you did this with a statue, the foot of the statue is not a single point.  It's possible that if the foot is flat, it would take only a tiny shift in position for the weight to be present on the inside of the foot or the outside of the foot.  These shifts can change the weight distribution.  This can cause your measurements on one side or another to be unequal.
