Generally speaking you usually get this message when you have a topic open in the RoboHelp editor, then perform some action that makes a modification to the file. Perhaps you executed a find and replace operation. The bottom line is that the file sitting on disk was modified in some way after it was first loaded into memory to make edits.
When I facilitate a RoboHelp class, I will regularly admonish folks to close all topics open in the editor before proceeding with any find and replace or other operation that will modify the files. Just so this confusing issue doesn't arise.
Helpful and Handy Links