Spotify came up with a sneaky one. I recently signed up and it only asked for some basic info. By default, of course, all email notifications are turned on. When you try to turn them off and save changes - it won't let you save because one "mandatory" field is incomplete [1]. So you're forced to add more info if you want to turn off email notifications. It's not a big deal, but it falls in the same category. I like t…
[I've encountered similar issues, e.g. where I had entered my home address on Amazon using a format which at some point started being considered "invalid" by their input-validation code. The old address remained in the system (and worked fine when ordering), but I couldn't change any other personal info without also re-entering that address, because the input-validation wouldn't accept the old one (it apparently still validated all the old data at that point, even stuff I hadn't changed). Unfortunately the new system was much more restrictive about what it allowed in addresses, so I didn't want to... >< ]