Almost never but .....
One time I had a few eggs under a broody hen and more in the incubator. That's the way I usually do it now. If something goes wrong with one of the incubations the hen still gets chicks to raise. Only two eggs hatched under the broody, both red chicks. These hatched a bit early, that's pretty normal with my eggs. The incubator eggs were a day later and included both red and black chicks. That broody accepted the red incubator chicks but not the black chicks, I had to raise those myself. I think she bonded with her two red chicks and knew the black ones were different. What is strange is that earlier that spring she had hatched and raised another brood that included both red and black chicks. I really did not expect her to bond only with the reds this time.
When this was going on I had another hen go broody a couple of days before these chicks hatched. I tried putting the black chicks under this broody to see if she would accept them instead of giving her eggs to hatch. She had only been broody a couple of days and would not accept them. She did not hurt them, they stayed under her and were not attacked, but she did not lead them off of the nest when she should have. When those chicks started their distressed peeping which I interpreted as they were getting hungry and thirsty I took them and put them in the brooder.
That's the only time I've ever had a hen reject chicks. I don't always wait for the hen to be broody for three weeks either. I have had chicks that initially rejected the broody. They'd try to wander off and leave her. The hen was pretty rough on them, pecking them to discipline them and forcing them under her. Within a day they had accepted her as their mother and stopped trying to abandon her.
Usually it goes very well but with living animals you don't get any guarantees.