My lesson is make sure to backup your wallet / keys. Luckily only lost a small sum but scared me enough to always make sure I have my wallet backed up.
This was true when i started bitcoin, had a few hard disk crashes and luckily for me i have taken a print out for my private key and so i was able to recover my coins and i have heard many loosing the coins because they forgot to take any backups. But now everyone is aware of the risk and a backup is necessary.