Even if you're not going to go the full monty and change the user-visible interaction, you can file off some of the sharp edges. For example, I have the traditional sign in and register forms, and (predictably, given that my customers are not too tech savvy and the forms look similar if you read none of the words on them which, after all, is the default behavior on the Internet) many people use the wrong one.

Since user intent is clear if they try to register a user name and password that are already in your database, rather than giving them a "that account already exists" error I just log them in straight away.