Rei

I guess I would say the paradox of tolerance. I’m sorry but I’m just gonna yoink the definition from Wikipedia because I’m not great at explaining things:

The paradox of tolerance states that if a society’s practice of tolerance is inclusive of the intolerant, intolerance will ultimately dominate, eliminating the tolerant and the practice of tolerance with them. Karl Popper describes the paradox as arising from the fact that, in order to maintain a tolerant society, the society must retain the right to be intolerant of intolerance.

Bonus least favorite paradox: You need experience to get a job and you need a job to get experience.

I’m not sure if this one counts, but I’m a fan of Rogule

Rogule is a minimalist online Roguelike game you play in your web browser. Everyone gets the same dungeon each day. You get one chance to beat each day’s dungeon. It is free to play.