I cannot look at a 16-digit credit card number and tell whether I’ve mistyped it or not. And neither can anyone else. I can, however, easily examine four separate four-digit numbers and spot typos.

1341329913245890 or 1341 3299 1324 5890? The choice is obvious. Yet most websites will not accept anything but the 16-digit string. It’s a trivial matter to strip the spaces and normalize the credit card number, and it speaks to the in­com­pe­tence of many website developers that they don’t do this. The cognitive burden should be pushed onto the programmer, not the user.

On a related note, Irish people write phone numbers as a seven-digit string. I can’t parse 6274986 at a glance either, but I can parse the US-style form, 627-4986.