Banks don't just go in and break their systems, if there was a software issue with their API they would roll back any recent changes and make it work again.
Haha. You don't work in IT, right?
I do but not in banking and when we get something wrong we fix it - and we fix it quickly.
There have been some high profile issues with software upgrades at banks in the UK over the last year or so - big banks. When they happen they are fixed very quickly.