Metro Bank has a great feature for transfer amounts. When you type an amount, commas are added automatically. However, if you paste a number with commas in it,

Metro Bank has a great feature for transfer amounts. When you type an amount, commas are added automatically. However, if you paste a number with commas in it, it will display correctly, but on the next screen you will discover it is transferring a different amount, as if it parsed the number up to the first comma. (As an aside, this is one of the underrated things about best-practice React: that kind of bug can never ever happen. What is displayed is fixed by the rendering of the data model, never the other way around.)