What happens if you connect two USB downstream ports with a USB male to USB male cable?

[Rodion15]

It may seem pointless, but there's a big chance that kids on a school computers connect a laptop to a desktop usb ports with one of the USB male to male cables around.

To clarify the meaning of Upstream VS Downstream I just did this summary after I clarified it to myself:

In telecommunications generally, a transmission from an information server toward an end user is referred to as downstream and a transmission toward the server is referred to as upstream.
My summary:
In USB terminology:
Upstream port: Receives power, for example the Micro-USB port on a smartphone is an upstream port.
Downstream port: Gives power, for example, the USB on an Apple keyboard is a downstream port.

A computer has a number of downstream ports.

On USB hubs, the port that connects to the computer is an upstream port, while the ports that connect to devices are downstream ports.

 

[Chicano]

With straight USB cable, nothing can be achieved... to connect two computers via USB you need a “bridged” (or “USB networking cable")... The bridge chip protects usb ports and since USB ports carry a current, directly connecting two USB ports with a simple USB cable can burn the ports or much worse, so it shouldn't be attempted.

Read more at http://www.hardwaresecrets.com/connecting-two-pcs-using-a-usb-usb-cable/#XLYDrRJW0tEwHG7t.99