Human beings are obstinate creatures who don't always heed the warnings of others. Which is why sometimes, the only way to really learn not to do something is to do it and experience the consequences firsthand. This may be the most painful way to learn a life lesson. But at least it usually sticks.
In a popular Reddit thread, someone asked: "What is something you did once and never again?"
1.) From Te-hole:
I stole a tester eyeliner from the makeup section at a store when I was 17. The security guy took me aside in a small room and "questioned" me, asked for my information, and to scare me, he said this would always be in my record and companies won't want to hire me anymore and I burst into tears and had a panic attack. He said he'll let me off if I pay double the price of the eyeliner, so I did and he let me go.