The advocate is overly quick to claim an opponent has made a fallacy when in fact the opponent has not.
Faux pas" comes from French, meaning “false step”. In English (and French) faux pas are embarrassing violations of social norms. So in this sense a Red Flag Faux Pas is the embarrassing mistake of accusing someone of making a fallacy, when in fact they have not.
Just because a red flag has caught your attention, don't be overly hasty in claiming a fallacy is being committed. You may be taking a "false step". Is the red flag you've identified the basis of an argument? If so, then it is likely a fallacy is being made. If the red flag is an addendum to an argument or point, or simply not part of an argument, then it may not be fallacious (it might simply be impolite and/or irrelevant).
Update: I asked a few people who blog about fallacies for their opinion on this type of error. J. Casey @ the The NonSequitur has identified this type of error as "specious allegations of fallacy". Gary Curtis @ Fallacy Files has just posted this response: Fallacy Abuse. As Gary says, this is not a logical mistake, but mistake about logic. I think they all add to the "discourse" (to use a word that makes me cringe).