This is a minor thing, but if you add a movie to a list and then click Add to List again without refreshing the page, it doesn’t look like it is on any lists. Then, if you try to add it a second time (maybe you scrolled down to read and then came back up and wanted to make sure you added it), an error appears that says “Doh! You can’t add a movie to your list”.
Video: https://imgur.com/a/IK5QG8p