I bought a dozen B O dayolds, ready sexed, some years ago and even 2 of them turned out to be males. They are particularly difficult to distinguish and are not obvious by their combs. With mine, I did suspect for a few weeks that I had a couple of cockerels and this was because they had noticeably bigger feet and lanky legs. But it was only when the sickle feathers appeared near the tail, at about 20 weeks, that I was absolutely certain.