There are so many...

The obvious ones:
• Any sort of pseudoscience (homeopathy, astrology)
• Conspiracy theories (9/11, moon landing)
• Expensive magic pills that make you slim
• Racism, fascism

Less obvious, subtle:
• "You can't judge other people's opinions"
• Any inadequate or naive view on free will (as far as I can see, the amount of naive free willers is far bigger than that of theists)
• Copenhagen interpretation of quantum mechanics (maybe not anymore?)

Probably still controversial or hard to avoid even with good epistemic hygiene:
• Rejection of cryonics
• Tabooing human sexuality