With my first order I got a lot of cockerels. The straight runs were cockerel heavy and even the pullets I ordered were not all pullets. Ordered four Barred Rock pullets and received two males/two females. I can't remember off the top of my head what the total numbers were.
My first order from MMc included 3 Dominique pullets. All 3 were males. They did make good on it with a promise to replace them for free the following year, because I special ordered them through a local feed store, and when I went in to pick them up, I pointed out that they were all roosters. The manager swore up and down, and contacted the MMc rep who said that those mistakes don't happen... Long story short, I got a written promise for exchange. Dominiques, or barred rocks should not be mistaken, as they are fairly easy to sex ID based on feather color alone. After several years of dealing with MMc, if I ever have to order again, I'll be using a different company.
I've had excellent accuracy in multiple orders from MMcM over the years, but nobody can guarantee 100%. On the other hand, our nearest Michigan hatchery seems to use some voodoo method; they are straight run every time, not sexed! It's also possible that the wrong chicks came to the feed store for you this year. Mary
I've had good luck with MM as well. All of the pullets I've ordered from them have been pullets and all of the cockerels have been cockerels (I usually order at least one with each batch of chicks). If you opted to get the free rare mystery chick, expect it to be a male though.
It really depends on the hatchery. What breed(s) did you order? One way to avoid the error issue is to consider ordering sex linked hybrids or auto-sexing breeds, but that pretty severely limits the options you have to choose from. I always go into it expecting a 10-15% error rate which is the high end most hatcheries will claim - that way when it comes in lower I am pleasantly surprised vs. being horribly disappointed if I expected 5% or less and got more of an error than that. Many hatcheries will actually throw in females as the "extras" - it just depends on what they end up with in a given week's hatch - after all, it is to their advantage to "surprise" someone with an extra female or two as it makes it more likely they will return as a customer for future orders.
Well, the good news is that with the Dark Cornish any errors will be obvious fairly early on since the plumage of that breed is so different between male and female birds - as the chicks start to feather in you will be able to tell any boys pretty quickly.