Yes, one of my two 7Ds has done this from new, over four years now... And I've seen it happen on various other Canon cameras.
Usually it's a very simple fix you can do yourself.
In the hotshoe, on the LH side (most cameras I've seen... may be on the other side on some models, I don't know), under one of the leaf springs, is a micro switch that's there to detect whenever an accessory flash is mounted on the camera and the built-in flash mustn't be allowed to pop up. What typically happens is that even though there is no accessory flash mounted, the leaf spring isn't allowing that micro switch to pop all the way back up. So the camera "thinks" there is an accessory flash mounted.
Usually this can be fixed very quickly and easily by using a mini screwdriver or a wooden toothpick, sliding that carefully under the leaf spring, starting from the open (forward) end of the spring toward the closed (rear) end, lifting the spring slightly and gently recurving it upward so that the micro switch can operate properly. Done.
In a few rarer cases I've seen reported, "gunk" gets into the hinges of the flash causing problems, causing similar problems. Those hinges can be cleaned as someone else described. I'd be very careful not to drip any of the cleaning solution (isopropyl/rubbing alcohol is what I'd use) down into the camera.
If you Google this, you'll find several websites with illustrations of the fixes mentioned above. (Also how to tighten the hot shoe's screws, should they ever loosen. That's another problem that occasionally occurs.)
The best solution, IMO, is to simply get an accessory flash and stop using the built-in. I almost never use any built-in, which is why I've never bothered to fix that 7D with the problem. Built-in flashes are underpowered, are slow to recycle, and can only be located in the worst possible place for redeye and ugly shadow problems (too near and directly above the lens' axis). They drain the camera batteries heavily too. No thanks! I use accessory flashes, usually on a flash bracket and connected via an off-camera shoe cord. That solves all the problems with built-ins.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
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