One of our team has experienced this twice so far, in different projects. The latest incidence involved two topics, whereas the previous incidence involved a dozen topics.
Her description of the issue:
1. RoboHelp inserted a bogus bookmark to adobe.com at the top of the page (like: my_topic.htm#http://www.adobe.com).
2. I could not even open those 2 topics in RoboHelp until after I deleted the bogus bookmark in Notepad. (See message below.)
3. Although our style sheet is still attached to it according to the Properties window, the formatting is completely lost.
The error message she mentioned above:
The file 'C:\<project_path>\my_topic.htm' is associated with a 3rd party editor and may be open in the editor. If so, please close the file in the other application before continuing.
When she was confronted with this issue the previous time, there were way more bad links than were manageable, so she just re-imported an older version of the files.