Write like a human: "Monzo’s approach here is so action-focused, in fact, that you’d be forgiven for not recognising it as an error at all: you'll need your phone for that."

Ask yourself:

• would this message still work for someone having a bad day?
• could this language trivialise the issue?
• are we helping users get unstuck, or just showing off?

Use active voice:

We couldn’t process your application because the file type you’ve attached isn’t supported. Please re-attach the file as a jpeg and try again.

The active voices tells us who is doing what, while the passive voice is ambiguous.

Give a clear way forward: for errors a user can fix, we should clearly state what they need to do.

• Check that your card details are correct, or try a different payment method
• Enter your name

When we handle errors with care, clarity and empathy, we not only help users move forward, we show that we value their time, their goals and their experience.