One of my pet peeves is the misuse of the argument: "We've always done it this way." It appears to be an attempt to establish a form of 'customary law', working on the assumption that since they've been doing it for years and it's still working, it must be good. But in reality the person's just revealing that they're not only doing it wrong now, no, they've been doing it wrong for many years, and aren't capable of critical thought.

Another is people who blame an entire group for what an (or a couple of) individual(s) of that group has done. E.g. people of one gender who think the entire opposite gender are bad people. It's supposed to be a protective measure, but instead it ensures that the bad behavior of one person continues to negatively impact them for the rest of their lives. If this person is so worthless, why do they allow them to set the tone of all future (lack of) relationships? Also, it allows the people who do this to avoid their own responsibility in their poor choice of partners.

There are a couple of others, that I can't recall just now. So I was wondering if you guys had similar pet peeves and if you want to share?