Quite a few times. For instance, I once thought that going out in the cold without a coat would give me a cold because I was taught that. They said that if it was below 70 out, and I didn't have a jacket or coat on, I would catch a cold. Then came the Energy Crisis of 1973. People were told to set their thermostats below 70 (they were told 68), and I didn't catch more colds that winter. And I heard fewer people, among those complying with that request, got sick. Since then, they lowered it to 65 during the day and 55 at night in 1979, again no excess colds. I keep my place at 55 in winter, and I think it actually improves my health some--myth busted.
Another health issue is with vitamin C. My grandparents told me that it makes no difference. However, in college, I started taking vitamins during the summer of my first year--and got fewer colds while on high doses of vitamin C. Then, one year I quit for a time, and started getting more colds (just like everyone else) while off the vitamins. This continued until I started taking lots of vitamin C to ward off tooth abscesses. Well, it warded off about 2/3 of the colds I otherwise would have caught. That was proof enough for me.
As for the Bible, it contains all the proof I need that Jehovah doesn't love anyone but His Fat Self, and that Jesus was doing the work started by Satan to attempt to liberate us from God. Going against everything anyone else had ever tried to teach me about God, I realized that no loving God could keep me celibate against my will. Satan might be able to try, but if God was so powerful, why in hell didn't He step in forcibly and do something about it? If, on the other hand, God Himself was to blame, it would in fact go on for life--my observation to this day. Thus, I could now see other cases, right in the Bible, of God's tyranny--Nimrod, King Saul, and all those God had killed simply for not worshiping Him.
I am currently finding out whether yet another "fact" I had believed is wrong. I was told that fluoride is good for your teeth, and most of us have been told the same thing. Yet, I looked online for "mineral hardness". I know that healthy enamel has a hardness similar to topaz, which is around 8 on the scale. Calcium fluoride, which is formed from fluoride, has a hardness of only 4. The metal probe they use to poke holes in your teeth has a hardness around 5, meaning they can make a cavity with that thing if your teeth are fluoridated but not if they are not. Plus, the glycerin in toothpaste prevents minerals from being deposited in teeth, sodium lauryl sulfate creates damage to enamel and leads to canker sores, and hydrated silica has a hardness of 7--sandpaper. I switched to tooth soap, and so far my teeth feel cleaner longer because there is no glycerin or propylene glycol for anything to stick to. If my next checkup in October is better than the one I got in April, yet another myth will be busted--that toothpaste is good for your teeth.
