After reading this article, some comments and a bit of research on Google I wouldn't be surprised if I unknowingly bought and sold fake Cisco at my last place of work (who have since gone under).
It was the only job I've had that involved dealing with "The Channel", despite working in both sales then purchasing there I'm still not too clued up about that side of things (it's boring, as you don't get to play with the things you buy) and I'm still quite niave about what goes on.
We were a Cisco Select partner who frequently got invited along to our local Cisco offices as they were trying to push us more and more towards Cisco SMB stuff, our customers included local police, local government, schools, colleges and installers. We had accounts with Ingram Micro, Azlan/Computer 2000, Micro P, but we very rarely bought Cisco from them. We usually ended up buying "grey market" stock from brokers which was often cheap enough for us to add our mark up and still undercut the distributors, but the thing I'm really wondering about is the dirt cheap "OEM" GBICs and SFPs we used to buy which we'd normally put at least a 300% mark up on and still be cheap, these were one of the few things that weren't stock dependant, our supplier for them always had a good stock of them and they were always dirt cheap so we always had a reasonable stock of them.
At the time I never thought about the possibility that anything we sold was counterfeit, but looking back I suspect at the very least the GBICs and SFPs were, none of our customers openly questioned why a small company was being able to undercut the likes of Ingram Micro, with some of our closer customers it was a case of "yeah, it's grey stock, but we pass the savings on to YOU", but most of it was don't ask don't tell.
We were just a small business wanting to play with the big boys, we'd get pricing support from Cisco for big jobs, but we'd tend to take their quotation, remove the prices, send it to the brokers and say "see what you can do" and they'd pretty much always undercut Cisco so for a struggling company who might go under anyway the gamble of buying "grey stock" that could possibly end up being counterfeit will generally pay off.