I recently worked for a small regional bank and balanced the debits and credits with the Feds. There was only one update to checking accounts. That was in the morning. As long as you were in the black once a day, in the morning there weren’t any “payment declined” fees. The only other ACH transactions between the bank and the Feds during the rest of the day where check imaging (and there were very few because no one uses paper checks much these days).
If you want to bring down a big chunk of consumer banking, preform a denial service attack on a company named FiServ. It processes about 35% of all retail banking data.
I never worked on that side ( I ran big call centers for retail banking and web banking support.
I used to hear about a botched batch from Fiserve from time to time.
Something everyone should be taught in school: how to balance an account. And how this stuff works—getting from one place to another. Some of it would stun the average person.