The other night I listened to my friend swear his way through the online purchasing process for concert tickets. He knew who he wanted to see, how many tickets he wanted, and his budget. All was going well until he got to a point in the journey that kept tripping him up, and the longer it went on the more frustrated he became.
As UX practitioners, these are the types of experiences we try to avoid; we would never knowingly place an obstacle in a user journey that would cause such frustration. In the end, he managed to purchase the tickets but not without some undue stress caused by not being able to read the distorted text in a plain old captcha.