Various reports of Duty Free Shoppers being detained for "shoplifting" and threatened with jail if money was not paid for "bail" has been surfacing for the last few months. I'd heard about it when I was last in BKK but thought it was a one off. However, it seems with the failing economy and dropping tourism, this new scam plot is becoming quite brazen in extortion of tourists:
http://news.bbc.co.uk/2/hi/asia-pacific/8154497.stm
It seems King Power duty free has had quite a few cases of "shoplifting" whereby items you think you've paid for were not but still put in your bag. That is the beginning of the ordeal, as depicted in the article. Always check the contents of the bag against the receipt and ask. A simply screwup of the staff can lead you down a big path.
The legal system in Thailand is intricate and if you don't know thai and have no thai friends, it can be a real quagmire. When travelling to any country, I always have my embassy citizen assistance telephone number on my cellphone and also on a piece of paper in my wallet. In Thailand, have the tourism police and embassy info at hand. They will usually have a lawyer and translator on call to deal with these matters. Immediately call your embassy (bewarned, your embassy may not be all that helpful if you're not anyone famous). Even American Express Legal Assist is pretty decent.
If your country does not have official representation in that particular country, my advice is negotiate a price and pay it! Don't argue or antagonize but don't look like you're afraid too. Staying calm and negotiating is probably the best thing you can do. Don't be like that UK guy who assaulted immigration staff in Phuket, only to be charged for assault on an official, after his embassy cleared up his passport problems.
I must add that corrupt officials aren't only a thai occurrence. I've had to deal with quite a bit in Central and South America, Africa and Eastern European countries. I still love Thailand and won't hesitate to go. I just won't go to King Power for any shopping 
