The Shadwell Shams
The Spitalfields Life blog has a brief account (with lots of pictures) of the so-called Shadwell Shams. These were supposedly medieval trinkets, specifically pilgrim's badges (tens of thousands of them), that a pair of forgers, William Smith & Charles Eaton (aka Billy & Charley), claimed to have found in the mud along the Thames. The pair did a good business for over 10 years, from 1856 to 1867, managing to completely fool most archaeologists. The article notes that, "today their Shadwell Shams are commonly worth more than the genuine antiquities they forged."