An example I found not long ago is with Google Photos. If you create an album, you can offer to share that album with other people, through their Google account, their phone number, their email, etc. If they accept the invitation, it notifies you and their little circle darkens. If not, it stays light, and still stays in your list of people you've shared with. There's no way to actually remove these unused invites, and these people just stick around as "ghosts" forever. The only way to clean this mess up, according to Google, is to turn off all sharing of your album, and then re-share with everyone you want to share it with, so they all have to re-accept the share invitation (and you have to manually enter in every email address, phone number, etc. again). There's no way to just remove one of these "ghosts", even though you'd think this is a very basic admin task. There's forums where people have complained about this for years, and there is a Google person on there who just complains about people using the forum for complaining like this instead of filing an "official" feature request (which as we all know never goes anywhere), even though this Google person could very well do this herself instead of complaining about all the complaints.