Stupid assumptions when collecting information about your users
An ordinary person tried to sign-up for an online service and was greeted with an error message “Your last name must be at least 3 characters”. The issue at hand is that her last name is “He”, thus she couldn't continue the sign-up process and likely had to fake her last name, or more likely – just hit Ctrl+W closed down the window.
Cultural bias is unfortunately very prevalent among UX designers & developers. In many areas of the world, it's not even common to have the distinction between first- and last-names, and in some places you're recognized by a single name. When collecting information about your users, don't make any assumptions about what cultural norms they adhere to. Our recommendation – and apparently w3.org's too – is to use just a single input-field for their name, thus your users remain in power to decide for themselves what they want to be named by you and your organization.