"someone directly edited the html output" substitution

I have several pages with this error in "Component Errors":

"Someone directly edited the HTML output of a FrontPage Substitution
component; the changes have been overwritten."

Every time I try to open an HTML file for editing, it displays this message:
"The contents of a FrontPage component have been modified. These contents
will be overwritten when you save this page."

These messages will not go away. What did I do and how do I fix it now?