Sometimes subtitles are not in the language they are meant to be in: for instance, you click on the "English" link in the navigation menu on the left, and the page that opens has subtitles in Spanish, or in a mixture of English and another language. This kind of thing usually happens when a user makes a mistake, either uploading the wrong subtitle file, or translating directly the original track instead of using the "Translate" link on the main page of the video.
In such cases, first check the revisions via the Revisions tab in the mistaken subtitles page: there might be a previous version in the correct language. If so:
Note: in the past, rollbacks sometimes triggered a bug that messed up other subtitle tracks and deleted the content of their own revisions, but this bug was eliminated in last Summer's software upgrade, so now doing a rollback is safe.
If there is no correct previous revision: