So far I can't think of anything that is being given a free pass (although I have no doubt there are many there). I think this is a great assignment and I will continue trying to ferret out some erroneous belief so that I may expose it to the light of day.
In the meantime here's something I discovered a few years ago. I consider myself something of an audiophile and one thing I used to accept without question is the superiority of high-end speaker wire. Companies like Monster cable claim superiority over your standard Radio Shack brand and charge significantly more. A few years back, James Randi offered Pear Cable Company a million dollars to prove their $7,250 speaker wire was better than Monster in double blind tests, but I don't think anything came of that. I am not aware of any large scale double blind tests that show either way, but these days I do question the validity and wisdom of spending so much money for speaker wire. It seems very likely that most people, at least, would simply not be able to tell the difference.