The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.
Stillborn: In fly fishing, a stillborn insect is one which got stuck in its nymphal or pupal shuck during emergence and floats helplessly on the surface instead of flying away. It is a specific class of cripple, although it is sometimes used interchangeably with that term.
This other word references the same concept: stillborns.