I just broke one of my own rules: I argued about vaccination with an anti-vaxer on Facebook.  Vaccination issues (and the vaccination opponents) are one of those topics that it is wise for me to avoid in person and online, because I can get nasty.  I managed to be civil, demonstrating the flaw in her argument and backing up mine with statistics from the World Health Organization.  Of course, the fact that she is a friend of mine helped in that regard.

Go me.