How have you lost Bitcoin through self custody mistakes?

I'm trying to get deeper into self custody in the light of the recent Ledger incident. So I've been looking into all sorts of exciting things such as running my own node, Electrum server, multi-sig wallets, using air gapped signing devices, even DIY devices etc. And then I thought, it would be sensible to ask for people's own stories of how they lost their Bitcoin through self custody mistakes.

As Otto Von Bismark said: “Only a fool learns from his own mistakes. The wise man learns from the mistakes of others.”

So, how have you messed up doing self custody in the past?

Seriously though thank you for the advice and wise words!