Floating points.
If you're at all interested in mathematics or computers or both, check out Joel Spolsky's brilliant explication of that new Excel bug.By now you've probably seen a lot of the brouhaha over a bug in the newest version of Excel, 2007. Basically, multiplying 77.1*850, which should give you 65,535, was actually displaying 100,000.
Oops.
The front page.