Ruined Hair
Girls these days know that hair care is more than just conditioner and haircuts. Back then people would straighten their hair on an ironing board at the hands of a careful friend. This is something that hurt quite a bit and could end in burns, so thankfully it's done now...
Chicken Pox Party
Before there was a vaccine for chickenpox, parents would throw pox parties with infected and noninfected children. Once everyone had it and overcame it, they would be safe from the virus forever. Unfortunately, what they didn't know was that it could return as Shingles later in life... Thankfully, once there was a less painful way to do it, people generally took that route.