The worst that can happen is that you could blow up the USB port by having a short on the power rails. There is a poly fuse to protect against this but these take a long time to blow and might not be fast enough to prevent damage from all USB ports. However we have herd of the occasional USB port going here but most of them shut down before too much damage has occurred requiring a simple reboot.There is no way that the arduino can hurt my computer right?
However if you go sticking mains voltages on your arduino you will blow that and probably take a chunk of your computer with it.
So "no way" is putting it a bit strong, but in general it is quite tolerant of mistakes.
