I found an old reference to this issue going back to FileMaker 11. What I found may or may not apply to your situation.
Are you using a Mac OS?
If so, might "Web Publishing" be turned on in System Preferences an Instant Web Publishing enabled in FileMaker Pro? (Open Sharing | Instant Web Publishing from the File Menu and check to see if check box is selected.)
If you are using a Mac, you may also be able to remove the preferences file For FileMaker 12 and see this issue resolved.
Here's the thread so that you can check the details for yourself, but keep in mind that this was for FileMaker 11, not 12:
For More Information see: http://forums.filemaker.com/posts/bc50c9fef9
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip