Illegal vs. Unwanted States

I've noticed this with UI interfaces before: For example, an IBAN field which actively blocks you from entering more than the expected number of characters. Seems like a good idea - except whoever sent me their account number helpfully included spaces to separate blocks of numbers, so when I copy-paste it the last few digits get cut off. Now I not only have to delete the spaces, but also copy the missing digits again! Just make the field red and block me from submitting until I fix my input!