I just found this usability pearl:
After entering a credit card number as usual with spaces between the groups of four digits, the web page complained like this:
Yes, it is easy to allow spaces. Just match the following regex
/^\s*\d{4}\s*\d{4}\s*\d{4}\s*\d{4}\s*$/
and then remove the spaces when processing it, but do not let the user enter the number without spaces. That is just ridiculous.
