I've never had this happen in Thailand. I do, however, usually only buy a few things and know what they cost. I've had countless times that restaurant bills, hotel bills and other stuff have been wrong. Often, it's hard to tell if they're actually trying to scam you, or not. Seems your case was pretty clear cut, though.

My biggest problems are usually with the minibar. If it's priced reasonably, I may drink one or two drinks during a weeks stay. Otherwise I don't even touch it. A few times in BKK I've replaced the drinks, but usually not. Getting charged for stuff that is missing is very aggravating. I actually got to the point, I'd open up the fridge and take a picture as soon as I got to the hotel, so I could point it out later that there were not 4 Singha beers in the fridge, but 2. (Plus I don't drink beer!)