On 23rd December 2009 I bought a pair of "16 GB SD Flash Memory Card **UK STOCK**" from eBay seller "tradedeals" (now trading as " eyelydelighted", his fourth eBay ID so far) to go with my SD-card based DVR.
They arrived, they seemed to work, I left feedback. Then the problems started.
They worked at first, but once a few hours of recording had taken place they'd corrupt and I'd lose the lot. OK, I put it down to speed, assuming the cards weren't fast enough, and left it at that.
Fast-forward to 21st July 2010 and I try recording some video on my Panasonic DMC-LX2, using the same cards. Same thing happens. In addition, copying files from my laptop to the card resulted in corrupt files as well. So I snapped it in half, and was surprised at what was inside: a Samsung K9G8G08U0M memory chip. Nothing unusual about that, you might think, finding a flash chip inside an SD card? Well...
THE K9G8G08U0M IS ONLY A ONE GIGABYTE CHIP!(Google the part number if you want to check yourself).
Yes folks, it's a 1GB card that thinks it's a 16GB card. This means that as soon as you go over the 1GB mark everything goes loopy. You can't even use it as a 1GB card because you can't format it as one. It's useless.
If you've bought an SD card and want to test its true capacity, Google-search for "h2testw", this is a program that thoroughly tests cards and will report any anomalies.
While you're at it, search for "fake sd cards" and check out the link to Steve's Digicam Forums, there's a lot of useful information there, including example speed tests.