When you lost to the guy in extreme high-speed physical combat, but then it turns out they were a summoning mage.
@Zathz
S rank is just the label they slap on things (Hunters or gates) who give off so much power that it exceeds the level where their measurement devices can read them. It's like, a power level of 9001 is over 9000, but so is 1 billion. There are just not many S ranks in the world to begin with and they're so far above other hunters that the general public just treats them collectively as the cream of the crop, but there's a lot of room for huge power gaps between them.
Though our MC probably puts most if all of them to shame by now.