Nielsen's Top Ten
Being the time of year when all and sundry come out with their "Top Ten of 2003" lists, Jakob Nielsen is no exception - see Top Ten Web Design Mistakes of 2003.
With most of my work (alas) at the minute involving detailed form based pages, you can be sure I'll be waving the text from point number 9 ("Overly Restrictive Form Entry") at every opportunity I get. Jakob's note about almost every credit card digit entry field not allowing groups of four with spaces - the same format as your card - has been a personal annoyance to me for many years now.
Which in "web years" is practically a lifetime, I believe.