What lesson did you learn the hard way?
the hard way ：〔副詞的に用いて〕痛い目にあって
When I was a child, I threw a stone high up in the air in a narrow alley
and the stone hit the windshield of a parked car.
The windshield got cracked and my parents had to pay for the damage to replace it.
Since then, I have never thrown a stone.
