What if immediately after you spent that $500k that you found in your account you called the bank/broker and asked them if the money was yours to spend and they said yes. Then, the following day just to be sure you call again, and again the answer was yes. You also asked if there was any chance you'd have to return the money and they said "No chance." Then you send an email just to cover yourself one more time, and in their reply they say "You're fine. The cash is in your account and you can spend it freely. There's no restriction whatsoever."
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