Everyone gets exasperated by bad websites. But we think of those things as tiny annoyances, and assume that users will eventually get what they need. Not true: Tiny mistakes can cost businesses dearly—in fact one poorly designed button might cost $300 million.
Jared Spool runs User Interface Engineering, which does usability studies on e-commerce sites. Recently, they did work  for a major retailer, whose site probably had the most unoffensive design you can imagine: After filling your cart, you press "checkout." After that, you're prompted to log-in or register, before finishing your purchase. The web designers assumed that returning users would know their info, and the new users wouldn't mind the little step of signing up an account, since they'd probably be back. An innocuous set of assumptions. But a massive mistake.
UIE studied people actually using the site, and it turns out that the prospect of registering was enough to turn some users away; meanwhile, even return users had problems logging in because they didn't remember the email address or password they signed up with. (45% of users apparently had multiple registrations—a few had up to 10.) Granted, these represented a small portion of users. But for a retailer with $25 billion in sales, even small portions signify huge lost profits.
So UIE redesigned the site, replacing the "register" button with "continue." They also added a message, saying that registering wasn't required to checkout, but was optional and might be helpful if you returned.
Sales went up 45%—$15 million in the first month, and $300 million in the first year.
Given UIE's success, it's a wonder how often you still see byzantine registration forms on websites. But there's a bigger lesson: That empirical research into how people actually behave matters far more than the intuitions of any designer, no matter how experienced that designer might be.
[Via Kottke ]