Some revisions have been merged under it that have not been mergedThis post on CollabNet's Subversion blog helped me out. The problem boils down to the fact that there is "subtree" mergeinfo in the branch and/or trunk which prevents the merge from completing. Usually you can safely delete the subtree merge info:
into the reintegration target; merge them first, then retry.
svn propdel svn:mergeinfo <subtree> -RI honestly don't understand why this issue arises or why Subversion is not able to resolve it. Despite having mergeinfo is much better than not having it, I can't help feeling that this is a kludge to handle a feature which Subversion wasn't at all designed for.