I’ve just read that there is a bug in Excel 2007 and if you multiply 850 by 77.1 it will give you the answer 10,000.  So in the course of scientific research I tried it and it does.  Change it to 77.2 and you get the correct answer of 65620.  Apparently any multiplication with the result 65535 is at risk, though I cannot find any other combinations – well in the 5 minutes that I tried.  Come on Bill you can do better than this.