Some revisions have been merged under it that have not been merged into the reintegration target; merge them first, then retry.Uh... which revisions? Merge what? Huh? @#%^&*@#&$^*@#&^!!!!
Googling this error message yields a bunch of conflicting advice. In my case, this thread was the fix. Use:
svn propget svn:mergeinfo --depth=infinityat the root of the branch working directory. You should get mergeinfo for only the root ("."). Delete the mergeinfo for all other objects (using svn propdel svn:mergeinfo) and commit.
Note that this is with SVN 1.5.4. I think perhaps it is fixed in 1.5.5???, though problem situations created in <= 1.5.4 will persist.