Unexpected empty cards when using cloze deletion
When using cloze deletion cards, it's fairly common to end up with empty cards—where you come to them in review and are told "the front of this card is empty." In all likelihood, this does not mean you have done anything wrong, nor does it indicate anything wrong with your collection. Rather, it happens when you create some occlusions on a card, then delete some of them. For example, you might make a c1 and a c2, then decide they should both be tested at the same time and change the c2 to a c1. This will then result in an empty card, because Anki has already created a second card for the second cloze, and in order to prevent loss of scheduling information if the change was accidental, Anki does not delete these cards automatically.
If you run Tools -> Empty Cards as the message suggests, you can delete these empty cards and get your collection back to how you expect it. This will not delete any notes, so you do not need to worry about losing data. If you are using the mobile version, you can bury the card that gives this message until you get back to your desktop computer and have the chance to run the Empty Cards function.