Having some experience with rolling mills, you can get periodic glitches in the machinery that will yield thicker or thinner stock. The QC samples are taken every so many feet per roll, and unless that bad spot is where they test, it may very well go undetected.
I will, say that even though it probably has no added value, the number produced would be relatively low. I don't how wide the rolls of copper stock are that planchets are made from, but the number of coins produced would be determined by how many planchets could be punched from the area of the roll that had the glitch. The longer the glitch and wider the stock roll, will determine number produced.
Dan