One of the Peace dollars I had packed away was visibly thinner than the others, the shade of the metal was just a teensy bit greyer than what we're used to... and the features were similar to what you would expect with a weak strike. Also, on the reverse, the rim had a peculiar raised edge, as if the strike pressure wasn't centered correctly. The final piece of incontrovertible evidence? It only weighed 19.7 grams, compared to an average of 26.7 that seems to be the standard for unworn dollars. 
An item like this would be easy to detect if it was examined, even cursorily. But if included in a bulk purchase, it could easily slip by undetected... as it must have done with me.
Now here's the scariest part. I haven't purchased Peace dollars in at least five years. These things have been around for quite awhile.
Caveat emptor! 
