When you barf in the middle of an opera, preferably all over your date. The more steampunk the opera, the more of a probability that wagnering will happen. The wagnerer typically climbs over other opera patrons, covered in his or her own vomit, attempting to reach the bathroom to wagner some more.
Girlfriend: OMG go to the bathroom the opera is happening
Patron: Wtf is that smell, did someone Wagner?!