Lemmings of Lemmy, You wake up one morning to find there was a bank error in your favor of $100,000. What is the first thing you do?

Honestly you report that shit to the bank if you know what's good for you, unless $100k is pocket change for you. If you go spending it, they will either get it back from you or you will go to jail for a number of years. Just about any court in the world will consider it the same as if you stole the money from the bank.

Just search and you'll find numerous cases where that's happened:
https://www.google.com/search?q=man+gets+million+in+bank+error

You move it to a high interest bank account and don't spend a penny. Wait until the bank contacts you asking for it back, you can then ask for proof documenting the error and that they require it back. This often takes quite a while. Once they've done it, return their money but you can keep the interest that was made on it.

It won't be a lot of money, but every little helps.

This reminds me of Patrick Combs who deposited a fake junk mail check "for the joke" in 1995 and actually received the 95000$ in his personal bank account. The story of what happened after is actually really interesting. Here is a good video about it (it is in french but there is good subtitles in english, and a lot of the video is in english anyway). He manage to write a book about it, became some kind of national hero representing the honest simple man vs the big banks, and became a motivational speaker, telling the story of his war with the bank, all the intimidation, fears, etc.

See if I can also win second prize in a beauty contest, then drive my iron to Vermont Avenue.

Do nothing. Banks in the US will claw it back.

I wouldn't trust it for a hot second. If there's one thing people do not miss, it's money. And if they are banks, they'll throw the book at you to get it back. I'd call them and not touch my account until it was sorted.

Check to make sure it's not a scam (those scammers are getting crafty these days)

The fictional antihero me would keep the money (and probably go to jail). The more likely result would be a phonecall "Hi excuse me but you seem to have accidentally transferred a large sum of money to me. I'm fully willing to return it, minus a 10% admin charge"

I agree with the reporting it, but id wait for the bank to realise the mistake. Do not spend a penny. If you can afford the risk and possible losses get it invested lol

Call them and tell them so I don’t get sued into oblivion after spending it…