You look in your bank account and there is 50 billion... Your first move?

I would buy weed and booze and go home and buy stuff on the internet. Pay my mortgage, make my current home offgrid and keep it for a base in the 'old world' so I can still see my old friends and family without them knowing about my fortune.

I'd then create a main plan and buy a military base and land in the US with a small town around it and massive walls with an automated defense like lots of automatic guns that'll popup for unwanted visitors all fully selfsustaining in case society goes down, I would make it a futuristic place with lots of tech.
I would select and invite people to come live there, basically it would be about education and self improvement, research, faith, sport activities and a lot of weed and moonshine. I would want to be like the Salvation army with homeless shelters across the country with free weed to care for the weak in society and get them up on their feet or as stable as possible. Followers can either stay at the main village and work or go out and participate in meaningful protests like activists or make video's for the internet with inspiring messages. Then have a public part like hotel, restaurant, dancing and pool where visitors could get high/drunk/party whilst being brainwashed by my followers lol

I'd leave it alone until the bank corrects the mistake. You spend $1.00 of that mistake, youre liable for it, and they WILL come after you for it

1. Quit my job
2. Pay off any debts my immediate family may have. Set up a trust for them for retirement.
3. Buy thousands of acres in Montana and build my dream home.
4. Send a pic of my dream home to my ex wife with a note: "Wish you were here" with my phone number, and when she calls answer and say "Not"!
5. Set up a trust to provide trained service dogs to people that need them.
6. Hire my very own Navy Seal team....to take care of...uhmmm.....stuff.
7. Mow my ginormous yard.