Today, I took delivery of some accessories (a USB WiFi dongle and a ModMyPi case) for a first-batch Raspberry Pi that's been sitting here for well over a year, waiting for a project. This RPi has never been booted up before now (I'd pre-prepared its SD Card in my main RPi, before the accessories arrived), and I temporarily hooked the machine up to my main RPi's Lapdock setup, to make setting it all up a bit more convenient.
Since I understand that early Model B units cannot be "back-powered" like the later ones, due to a polyfuse which limits the current that can go through the USB ports (is my understanding of this correct?), I connected the RPi to a suitable known-good external power source, and assumed that it would only be able to use the Lapdock's USB interface to make use of the keyboard, rather than for power as well.
Anyway, this seemed to be the case, and I then got to work with the WiFi dongle setup, which was ridiculously easy.
All was going well, until about five or ten minutes later, when suddenly smoke (fine, whispy, white smoke that didn't smell of anything - so, I don't think it was Magic Smoke) started pouring out of the area around the micro-USB connector and the SD Card slot! (Mostly the micro-USB connector, though.) The Raspberry Pi didn't seem to have noticed this at all and kept happily chugging along, but I immediately yanked the power cable out, and unplugged the RPi from the Lapdock, too, and quickly pulled the case off to find..... Absolutely nothing.
The board was clean, there was no sign of any reason for it to have been smoking*, and nothing was hot - the board was simply the "lukewarm water" sort of temperature you'd expect to find in running or recently-running electronics.
*All I did spot was a tiny little piece of paper/fluff that for some reason was stuck to the back of the SD Card slot (I took the RPi out of its bag and put it straight into the case, so it must have come out of the factory like that; I didn't notice this at the beginning). I have no idea if this touching the SD Card slot would cause smoke - it didn't seem hot enough to burn paper/fluff, at all.
I cautiously plugged it back in, and to my surprise, it was absolutely fine, as was the SD Card (no corruption or anything). It didn't start smoking again this time, but I also didn't run it for as long - only for long enough to check that it was still working, and that my just-completed WiFi setup hadn't been lost when I cut the power.
I have a suspicion as to what apparently happened, but would like to be sure before I put this thing in the room where I'm intending to set it up; I noticed when I unplugged the power but before I disconnected it from the Lapdock, that the unit kept pinging between being off and the rainbow screen. It looks like it was, regardless of the fuse, taking in a little power from the Lapdock via the USB port, which I didn't think it was supposed to be able to do.
Is this apparent taking in of too much power likely to have been what caused the issue, or would it have done this anyway for a different reason that I'm not aware of?
Does anyone know if this has happened to any other early Model B units, and what the cause was?
Or is this an indication of a fault with this now-out-of-warranty unit, meaning that I really shouldn't use it if I don't want to risk it doing this again?
Any pointers and insight will be greatly appreciated - thanks very much in advance to anyone who might know something.