Security

"Security" in Bitcoin has two distinct meanings that often get conflated:

Protocol security. The Bitcoin network's resistance to attack. This rests on proof-of-work, a globally distributed mining industry, every full node independently enforcing consensus rules, and a sixteen-year track record. The network has never had a successful 51% attack on its mainnet, never had a double-spend that overturned confirmed transactions, never been censored or shut down. The cryptographic primitives (SHA-256, secp256k1) remain unbroken. The protocol is the most-attacked cryptographic system in the world; it has held.

Operational security ("opsec"). Your personal security as a Bitcoin user. This is where the failures actually happen:

• Seed phrase leaks. Photographed, written next to a computer that got hacked, stored in a cloud drive, shared with someone trusted who became untrusted.
• Weak entropy. A key generated with predictable randomness. Has happened many times; people have lost millions to it.
• Phishing and social engineering. Fake support staff, fake wallet apps, fake "wallet upgrade required" emails. The dominant way ordinary users lose Bitcoin.
• Custodial failure. Exchange insolvency, hacks, freezes. Real losses, real customers, real bankruptcies.
• Compromised hardware. Supply-chain attacks on hardware wallets, malicious firmware updates, side-channel attacks.
• Inheritance gaps. Dying without a usable inheritance plan is a permanent loss event.

Protocol security is what Bitcoin gives you. Operational security is what you have to give yourself. The defenses (hardware wallets, multisig, air-gapped setups, careful seed management, avoiding custodial honeypots, KYC-minimization) are well-known. The discipline of applying them isn't automatic.

A reasonable framing: Bitcoin's protocol layer is secure to a degree that's unusual in computing. Your wallet is secure to whatever degree you make it. Don't confuse the two.