Seriously? If the rule includes the fact that I can't post anything before/after to indicate it was part of a game or bet, then there are plenty of things I wouldn't tweet for $20, or a fair bit more. Setting aside work-related issues (that exist for me but maybe not you), would you be happy having people think you are racist, homophobic, or anything else along those lines that would make people think different of you as a person? There's most possibilities to "embarrassing tweet" than just "I eat poo".
Forget embarrassment: there are things you could post on Twitter, like a threat to blow up a school or a government official, that would land heavily armed people at your door to drag you off to prison on felony charges.