I realize this amount is excessive, but I have seen several occasions where Social Security gave a COLA or something of this type, and sent out checks or hit bank accounts with retroactive benefits, for example the person in July finds that he received a COLA increase, and it was retroactive to January of that year, and suddenly, there's a large amount of unexplained cash in his checking account from Social Security, which he is afraid to spend, but he is legitimately receiving it, it's the backpay of that raise.
The important thing for you to do is speak at once with someone in the Social Security administration. Let them know immediately what has happened, make sure they haven't just misplaced some zero or decimal.
As to what would happen if you rapidly pulled it out and bought yourself a new car or something, I can personally say that I have seen cases in which people were paid benefits to which they were not entitled, say they did not live the entire previous month. When Social Security is notified, that amount is sucked back out of that bank account with remarkable rapidity.
If you managed to get in there and get it all out in cash before they got around to taking it back, I suspect, and have seen it happen that the first thing they would do is suck your bank account totally dry, with all the incurred overdraft charges and bounced checks and fees and penalties, and then they would mount an all out campaign to recoup their money, and do not forget, these people have access to lots of your personal information. So they could get your savings, your home, your personal property, earnings, lots and lots of ways they'd have to get their money back, as well as prosecuting the daylights out of you. Messing with Social Security is sort of like robbing the post office. It is NOT worth it for the severity of the penalties that would be incurred. Don't mess with the Feds.