I am convinced that the absolute worst case of UX fail is a form wherein, if you submit an invalid input, the form is wiped clean on a page refresh with a little red error above it, "invalid password." Well, thanks champ, I know, I hit the J key instead of the K, but now you want me to insert my email address all over again? Ok, I'll type it all in once more (why didn't I just click "save password information" the first time I logged into this damned site), but, oops, I thought my password started with a capital letter and it turns out it does not. Once again, the form refreshes. By this time I am so frustrated I might just close the window - not so good for retention, eh there buddy?
Who hasn't experienced something like this before? Why is this flow still rampent on the web when there are plugins like garlic.js to save form data in the user's local, or AJAX form validation that doesn't force a page refresh?
The latest culprit I encountered was the Asana internal interface - when trying to enter a new email address so that I can send updates from one project to a different account, I had to first enter my password. After three incorrect attempts I went searching for a "reset password" link and couldn't find one! That small annoyance leads to huge losses in the overall user experience, and probably their metrics as well.