1) MECP certification. Not so much the test itself (though given how many who are MECP certified lack basic knowledge of audio/acoustics, I'd have to say their test and/or material is in need of a serious overhaul), but moreso the implications people draw from it. Many who are MECP certified think it gives them a trump card in any discussion, and they think it means they absolutely must know more than anyone who is not MECP certified.
2) People who claim to be from the "old school" days. As above, they for some reason seem to think this is a trump card in a discussion, or think it means they must know more than someone from a younger generation. And generally it seems these "OG" enthusiasts are the most misguided and continue to believe in audio myths and magic pixie dust.