Sexism, racism, sexual harassment, requiring employees to do unethical things, physically endangering employees, taking advantage of mentally challenged employees, and religious discrimination are all things I've been screaming mad at employers for.
For me, the thing that made me most furious was an employer who took advantage of a mentally challenged employee at the shop where I worked. They took advantage of her financially, telling her things like that she didn't always have to get paid for time she worked and that overtime was only paid out for hours over 80 worked in a row. It was not a case of her labor lacking value, because she followed directions religiously, cared about the customers, and worked very hard. She was my favorite assistant because she never did anything half-assed and listened to me carefully, making her far more efficient than anyone else.