The WHOLE BANK!
A few years ago I was a teller. Every night we balance our drawer and bank our branches 'holdings' to a ledger.
About three weeks before, we had rolled out a new operating/banking system.
Going from an old 'green screen' to a Windows based one.
This one evening. I was the sole operator in our branch left to bank my funds to the ledger.
I did that, locked up ,alarm on....and off I went home.
The rest of the branches also did their banking nation wide.
The next morning I turn up to work to be told that there was a major system outage and no-one could collect their funds to start the days banking.
Every branch across Australia was unable to release the banking from the ledger.
We had to resort to our emergency operations. Manual transactions and cash withdrawals limited to a certain $$ amount per customer.
The word was the top dogs were screaming blue murder!!! IT couldn't figure it out.
Then it was discovered.
A certain branch.... had deposited an amount to the ledger....
And it had a double decimal in it.
That is all it took to bring the bank to a stand still.
An extra decimal point.
All they had to do was reverse the deposit and re-enter it.
Let's just say I had a stern talking to.
On the upside IT fixed up the formatting so you couldn't do a double decimal again!
EDIT. Not a major bank. Only our branches were affected.
"A few years ago"