Excel 2007 bug
I've just read on Slashdot about an error in Excel's multiplication. Details can be found in this Google group thread. In short, it seems a multiplication whose result gives 65,535 (which is 2^16 -1 or 0xFFFF in hex) will return 100,000.
I haven't tested it myself yet, but it seems several other people did. I would be worried if I were using that software for any calculation.
Update: I've already tested and it happened indeed (see the picture). Cell A1 shows the offending formula and A2 shows you the result of that formula (which should be 65,535 instead). However, I've tested other multiplications whose result should be the same and they all seemed to work ok. That's funny. (BTW, decimal point is "," in my locale in case you are wondering).