USB is keyed so that it can only be plugged into a port a certain way, to ensure that the contacts touch. But what if you either forced it to plug in improperly, or the port itself was worn down and the plug was worn down and you could plug it in backwards? What kind of havoc, if any, would it cause?
It just wouldn't work.
The contacts are only one one side of the socket/plug so putting the plug in the wrong way round will mean that no connection will be made.
You'll also potentially damage the plug and/or socket so it can't be used again.
I should just add (to answer the final part of the question) that Windows wouldn't warn you because it would have no way of knowing that anything had happened.
PQI has a habit removing the outer metal from the USB connector on their USB drives like the one I have above.
It's just as easy to put in "backwards", but as others have pointed out, it simply does not make contact with anything. Thank goodness there's a light on it, so it's easy to tell when I've put it in the right way ;)
Well the electrical connections would not match up, so I think the worst case would be a short which would likely damage the interface circuitry on the mobo. I doubt it's designed to handle that, because the connectors are keyed. Other than that I think it would be limited to the physical damage to the connectors i.e. they wouldn't work any more.
How persistent did you have to be to get that connector in backwards?
USB 2.0 requires that the all trancievers are able to withstand a short-circuit. Unless you have a truly substandard USB implementation it should be fine. It just doens't work backwards.
A fun story is that Microsoft used to torture-test USB by plugging a fork into the socket instead of a proper USB device.
Like many already said, usually it's hard to plug it in without force.
However, some manufacturers do have those tiny models, like this Transcend T3, which can be plugged backwards. But, since the contacts are only on one side, so no contact is made, nothing happens. Windows doesn't show any message, since practically, it's not plugged in.
Apart from that, and the fact that I lose them all the time, fantastic little thingies :)
If you connect the usb cable backwards on the motherboard, you could damage your pendrive. I saw this happen. This is a similar case of yours.
Sidenote: I accidentally inserted the headphone's jack into my usb slot (the case is on the floor, the slots are near to the floor and I was blindaiming). Some sparks came out and the OS warned me that something happened to the usb. I don't know if it is working now, because I didn't put in anything after the incident.
If the plastic spacer inside the PC's USB type A socket breaks, attempting to plug a device into the socket can short the Vcc and GND terminals together, which may damage the motherboard. Breaking the plastic spacer was not unheard of with clamshell cases on Dell Precision Workstations from several years ago: open the clamshell with a device plugged into the front USB port and something's probably going to break (either the device or the socket).
Depending on the mechanical design of the specific type A plug, fumbling around with the plug backwards may also short Vcc to GND. I ran into this once with a type A plug where the shell had two metal fingers that were crimped over the plastic spacer, and these happened to be the same distance apart as the Vcc and GND terminals. This burned up a trace on a motherboard (a generic Pentium board circa 1996; apparently no overcurrent protection for the USB port) and made me much more careful about plugging in keyed connectors.
An USB connector has just a metal case around it for protection. It has four connectors which will connect the device to your computer. Two of them are just to send power to the device. The other two are to send data to your device. Worst case scenario oif you mix these up? You'd be sending power to the data line, causing it to overload damaging the device. But if those connectors are mixed up, the device is very likely already broken. The mini/micro plugs have a fifth connector, just for grounding the device.
If you open an USB cable, the red cable will contain the 5V power. The white and green cables are for data. The black one is to ground the device so the power can loop around. With a mini/micro USB plug, the fourth connector is used to make distinction between the kind of plug. (USB-A or USB-B.) And... Oh, well... Wikipedia says it all. :-) No use to type everything over from there...