Here’s some simple product design inspiration and Tumblr advice from Dan Leveille:
On Tumblr, if you mistake the registration form for a login form and accidentally enter your existing username and password and hit the enter key, Tumblr catches the mistake and conveniently logs you into your account. There’s no point in displaying an error!
This highlights how making a smarter registration form can improve user experience. It’s an essential approach to consider with any interaction: let the system do the hard work and make everything easier for the user.