Adding my owner answer here thanks to some input from @Wats0ns and @Murch.
Let's say the following happened:
- I generate a key pair that collided with an already existing address.
- That address happened to have a balance.
- I signed a transfer to a new address under my (and only my) control.
- In that transfer I put my contact information (and proved after the fact that I had the private key and had done exactly as everyone suspects, crazy I know).
- The other owner to claim they owned the address via a digital signature (the only way you can claim to own an address).
- The other owner to show either that they owned the previous incoming addresses or that they traded with others to result in the funding of the address in question.
- The other owner to sue me for those coins back, and press criminal charges for theft.
- I would use a "Claim of Right or Ownership of Property" defense for the theft charge.
- I might lose the civil case and have to return the BTC.
For the theft charge, my argument would effectively be, "I thought I had just as much right to the coins as they did. We both generated a key pair, and the key pair is how you 'own' an address or any BTC the ledger says belongs to that address. I didn't act maliciously in generating the address, I didn't hack a computer and steal the private key, I just happened upon it."
I think that might be a reasonable amount of evidence to show "I thought it was mine" and hopefully that'd keep me out of jail.
I have no idea what a court would decide in the lawsuit as I don't know of any case law that comes close to this. Maybe it's like a bank accidentally adding you as a co-signer to someone's bank account (not heard of any cases involving this)? Or making an invalid deposit to your account (which does have case law, and you have to give the money back IIRC)?
Either way, I'd be willing to bet that the BTC would have to be returned, but it could be a coin flip...