Things employers do that are against the law:
Telling you that you can't discuss your salary with your co-workers. The National Labor Relations Act says that employers can't prevent most employees from discussing wages among themselves. The reason for that is that employees can't effectively organize or unionize if they're not permitted to discuss wages or uncover potential inequities.
But despite the law, an astonishing number of employers have policies against this anyway – so many that most people think these policies normal and have no idea that they violate the law.
Think about this during merit talks this year.... I've always been told to not discuss raises. Even Aberle told me this. This year I'm telling everyone what I'm ranked and started a spark group to discuss.