If you are using a simple href in your code but forget to close the quotes, the field and entire display becomes unusable and you need to delete it and start fresh.

Example code that causes this error: <a href="link-url>This is my link</a>

The popup window doesn't appear when you click on the field to edit it. Instead it appears below as a giant link. Clicking on the text area to edit this code acts as a link and you are unable to edit it.