When using the Table of Content macro along with the Content by Label macro on a page, the Table of Content macro will pick up the Header "Content by Label" if the macro is empty - no pages with that label.

To reproduce:

• Create a page in Confluence
• Insert a Content By Label macro with one label which no page is using
• Insert a Table of Content macro
• Click save, and you will see "Content by Label" (pulled from the text when the macro is empty)