Why Monzo’s bank transfers weren’t working on the 30th of May

This lets the receiving bank know there was a request message they didn’t acknowledge, and they shouldn’t put the payment in the recipient’s account as the sending bank has been given a rejection code. Our third party Faster Payments Gateway begins corrupting approximately 25% of all payment messages sent through it, in both directions. This is already fixed by our change to remove corruption from inbound payment messages, but some customers were left with money in their Monzo account that didn’t actually leave the sending bank.

Source: monzo.com