Mostly the mistakes that we do are in the early stages when we first learned about bitcoin and how it works.

I have lost bitcoin one or few times maximum can't remember well but how I lost them was that I forgot the password of the freebitcoin faucet.

Lost also the email access don't know how maybe my pc was infected few years back but the amount wasn't large but it worth a lot to me at that time because faucets are very time consuming.