As they say, a lie can travel around the world before the truth gets out of bed. I recall telling my Mom that you can't go swimming for an hour after eating. She said, "You're not doing laps for the Olympics. You're just playing in the water. Go on." And she was right, even if that didn't quite debunk the myth. We call them old wives' tales or urban legends, but there are people who will argue with you about these myths today. Some may have once had a reason behind them, or at least faulty reasoning that seemed to make sense at the time, while others are just made up of whole cloth, like eating spiders in your sleep.
You know what can make you feel old? I can remember when some of these untrue "facts" were considered "news" or at least new research findings. See all 14 myths that aren't true at Cracked.