This week, redditor /u/Uthayne posted that he had bought counterfeit Scalding Tarns on TCG Player. The evidence is conclusive.
Be careful when ordering online. If your card is worth more than about $30, make sure you test the card to make sure it's genuine. The light test is a very good quick and dirty test to see if a card is genuine.
However, not to worry. The fakes are not getting that much better. This card apparently felt much different and the typeface looked off.
If you do buy a fake card on TCG Player, contact them and they will investigate. If the seller is knowingly selling fakes, they will be dealt with.
It's impossible to really examine a card you buy online, so understand there are risks. If you're a seller, make sure the cards you send out are genuine by doing these tests yourself. We've seen cards as cheap as Hallowed Fountain faked, but if you send out a fake Tarn and you're on the hook for a refund and you're out the card, you could have a bad day. Protect yourself as a seller and as a buyer.
Fakes are still very easy to detect and this should not make you panic. Just know that this is a thing that happens and that the light from your smartphone is one of the best tools you have to detect this rash of counterfeit cards. Protect ya neck and buy in person where possible.