Suppose you gate-crash an IT Job Fair...

[PeregrineBF]

USB won't deliver enough current to fry a motherboard.
USB devices need to negotiate with the host to deliver more than a certain amount of power, so you'd need to have a working USB device request the power increase, THEN route it across the data pins.
I have, however, made etherkilller-style devices for USB. That WILL fry the motherboard quite nicely.