It always kills me to hear people say that they don't like or accept the non-aggression principle. Now, typically, it is opposed because they reject the definition of aggression - or what they believe is a lack of a definition of aggression, despite the principle defining it. You'll often hear them say, "Sometimes aggression is necessary." And if they say that while fully comprehending the definition and principle, that's your cue to avoid them.

What really gets me is when self-identifying anarchists reject the principle because they believe it may be necessary to use violence to stop someone who may potentially harm someone else, which is their best Minority Report-esque (violent statist) solution for dealing with thought crimes. Such arguments make me chuckle, but only to mask the sadness that I truly feel for them.