"Affirmative action is racist!" True if you define racism as "favoring people based on their race", but though the archetypal case of racism (white people keeping black people down) has nothing to recommend it, affirmative action (possibly) does. In the archetypal case, decisions are made based on race, success is completely decoupled from merit, and disadvantaged groups are locked into a cycle of poverty with little to no escape. Affirmative action keeps the first disadvantage, arguably escapes the second disadvantage depending on the setup of the particular program, and positively subverts the third. The question then hinges on the relative importance of these disadvantages. Therefore, you can't dismiss affirmative action without a second thought just because it's racist and you can dismiss most cases of racism without a second thought. You would also have to demonstrate that the unique advantages of affirmative action which other forms of racism lack fail to outweigh the disadvantages affirmative action shares with racism in general.

I think most contemporary invocations of "That's racist!" are examples of the worst argument in the world so I'm not so sure about that.