Also known as Authenticity Police and Authenticity Nazis and other snotty terms.
Make no mistake, these terms are insults. While they can be used to refer to someone who attempts to do things accurately, they are more often used to refer to anyone who is perceived to be trying to force their standards of accuracy on others. In the legends spread about them, they are frequently depicted as self-indulgent cretins who are lacking in some aspect of accuracy themselves. They are thought to frequently refer to people who aren't as interested in authenticity as they are as "Farbs" and "Bobs". They are believed to aggressively attack and insult others around them. These legends serve to create more trouble by causing people are already insecure about their level of accuracy to assume that anyone discussing their clothes, whatever, are in fact attacking them.
These people do exist in the SCA but are less common than supposed. They are, however, extremely common in UK re-enactment groups, where they constantly patrol seeking to ratchet up the authenticity standards to ever higher levels, and loudly criticising anyone who, in their opinion, does not come up to the "correct" standard. Most of them are inadequates seeking some justification for threir existence.