It may also be the case when you have combined the trunk or some other and then forgot to commit -
In my case, svn diff showed:
--- . (revision 2723) +++ . (working copy) Property changes on: . ___________________________________________________________________ Modified: svn:mergeinfo Merged /trunk/CrowdRoamingV2:r2667-2722
You can see that th now contains merge information in this thread.
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