The paradox of tolerance states that if a society is tolerant without limit, its ability to be tolerant is eventually seized or destroyed by the intolerant. Karl Popper described it as the seemingly paradoxical idea that in order to maintain a tolerant society, the society must retain the right to be intolerant of intolerance.
I have always found this paradox interesting. In Sweden, you have freedom of expression until your expression drifts into hate speech. Once it drifts into hate speech, it is then illegal. But who decides what hate speech is?