Not sure if there is a mechanical issue with the RaspberryPi USB connectors, whether I've just been unlucky, or maybe I'm getting clumsy in my old age.
Since the introduction of USB ports, I've been presented with computers to repair from time to time with mangled USB connectors. The most common problem is bent or broken contacts. I've generally assumed this to be the fault of the user, probably due to rough treatment or attempted insertion the wrong way up.
However, when I got my first RaspberryPi model A a few months back, I noticed the USB was very tight when plugging in peripherals. Within a day or so I'd managed to bend one of the contacts. I changed the board USB connector for one purchased from Spiratronics and its been fine ever since.
I'd just about forgotten this experience until yesterday, when a second Model A arrived. I plugged in an Edimax wifi dongle and the Pi failed to boot. Same problem. The two outside connectors were doubled back.
Now before you write me off as a knuckle dragging Neanderthal, just let me say I also have 3 Model B Pi, some of which have had the same devices connected to their twin USB ports, with absolutely no problems. I regularly handle many other model B boards which are used by our Computer Science students. Also, when I plug my Pi peripherals into my laptop, they go in easy but are firm when fully inserted.
So my conclusion is that the single USB connector fitted to the model A is at fault. But I'd love to hear from anyone else with similar problems.