My favorite question to ask Libertarian types is whether it is ethical to perform CPR on a stranger who stops breathing and passes out without advanced warning. Lots of Libertarians say it's always unethical to take someone's money without consent, even if they recieve something they want in return.
Okay, sure. But is it okay to break someone's sternum without consent while performing proper forceful CPR? Most Libertarians are just Communists or Capitalists or Conservatives and just don't want to present themselves that way for some reason. However, when talking with the rare exception, the CPR question often stumps them.