I discovered rather inadvertently that laptops do not enjoy having their USB VCC shorted to GND one bit. It is a sure DoS, in fact if the machine has a stupid power supply, it could result in permanent damage. It is kind of scary for kiosk machines like these Kodak kiosks.
I give you, the usb pwner. It could even hide inside a legitimate stick:
USB Pwner schematic VCC ------------ D+ ---(nc) | D ---(nc) | GND ------------