[trothamel]

I don't think that's possible, is it? My understanding is that the miners decide which of two competing valid transactions is the "real" one, in the case of double spending.

But miners don't have the power to cause an invalid transaction to become valid, if they were to insert an invalid transaction in a block, the block would be rejected by the rest of the network.

[matthewbauer]

Correct. You'd need the private keys to take someone's coins. You could still double spend though.

[RyanZAG]

Yeah that's right, you're correct.