We have faced this issue on a number of occasions, with the last occurrence being a number of reels of FETs from the authorized US disty for the FET manufacturer (fortunately). The parts were consistently failing even though they were used well within their envelope. Suspecting foul play, we called-in the rep who called in the semi house, who ultimately sent the reels to their lab for analysis because the parts were so amazingly (cosmetically) copied. Consider yourselves forewarned!
Beware of Counterfeits -- even if you buy from an authorized source