Theories / urban legends that have been repeatedly proven/disproven, or whose opinions change drastically.
Saturated fats bad, trans fats good. Saturated fats bad, trans fats worse.
People swallow 8 spiders a year
People listening to War Of The Worlds on radio thought it was true, causing mass hysteria. Nope. Effect was greatly exaggerated.
Woman sues McDonald's because she gets burned by hot coffee. Sue-happy Americans, need for ridiculous/obvious warning labels, etc. Turns out she had 3rd degree burns and was hospitalised for 8 days. Also, McDonald's has had numerous complaints about their coffee being too hot.
Stanford prison experiment - to prove there are monsters in everyone, take people, assign some as prisoners and some as jailers, see jailers abuse their power! Meant to show how normal people did evil during the Holocaust. Later proven that their methods were unfair and meant to encourage their conclusions. https://www.reddit.com/r/askpsychology/comments/3k8ob0/is_the_stanford_prison_experiment_taken_seriously/
Vacuum cleaners - first they used reusable cloth bags. Cloth bags are messy and unhygienic! Use disposable paper bags! Now paper bags are not environmentally friendly! Use reusable bags!
Government HDB apartment blocks in Singapore - in old blocks, rubbish chutes were shared and located in public areas. Then they switched to individual chutes in each household. Now they're back to communal chutes again.