I may be missing something that will be obvious to everyone else, but in this game I kept gaining "Band of Misfits" through the use of "Lurker", and the game kept letting me use BoM but then "returns a Band of Misfits to the Band of Misfits pile." I don't see that there is any reason for it to do this so I am wondering if it is an error in the card mechanics.
Cards used were:
cutpurse
moneylender
spice merchant
archive
band of misfits
royal blacksmith
encampment
lurker
masquerade
familiar
labyrinth
You were playing Band of Misfits as Encampment. Encampment is returned to the supply at the end of the turn it's played unless you reveal a gold or Plunder. This carries over to the Band of Misfits as well.