is it going to harm my pc
No, a decently designed USB port incorporates resetting fuses that protect everything
A decently designed - yes. A poorly designed will just reboot your computer and die. Forever. How to find out if yours is decently designed? Short it... If you follow the advice above, I'm not responsible for the loss of USB ports, motherboards, life, etc. including (but not limited to) universes.
AFAIK, fusing is part of the standard. Its too easy to imagine a peripheral or cable going short circuit.
I sold someone one of those silicone waterproof flexible USB keyboards for their notebook. It worked for a short time and then it shorted out. It actually smoked, then it burnt out the USB in the notebook. They ended up having to get an express card usb adapter to be able to use USB. They were not happy. I still have a couple of those keyboards kicking around, want to try one? Brand new, still in the box? They are perfect for nasty environments , like for in a shop, because you can get them all greasy and then just wash them. They are also good for clinics where they need to decontaminate things. Great idea, and up until then never had a bad one.
I could check it, although what i expect happened is that the flexing caused wires to shift around, and touch each other
True, but if you can sell something a little bit cheaper... A fuse might be cheap, but it's not free. And if you sell by the thousands... E.g. see this 'USB-charger': http://hackaday.com/2012/08/15/buying-cheaper-electronics-and-not-saving-money/
If you're that paranoid about it, you'd never plug in a mouse, or a harddrive or ANYthing !
He he, true... But the fact that I'm paranoid doesn't mean a short wouldn't kill my USB! :-) No, I will plug in any mouse/floppy/CD drive/HDD/whatever without giving much thought. I'd just make sure that anything I have constructed and built (or got from a trustful source [e.g. not that super cheap Chinese gadget vendor]) so to make sure it, a) has no short (tricky), b) has an reliable fuse (open it and check how it is built) or c) has at least some resistors to make sure my laptops motherboard doesn't get fried in a split second. For anything made and bought from a well non company - plug it in and be happy (or sue the heck out of them). For anything else - you're a hacker. Open it up, have a look inside.
Are you planning on intentionally shorting it?