While merging branch back to trunk I constantly get the error in the subject.
Note that x/y/z has svn:mergeinfo property. The property was not added on the branch, but it was already there when the branch was created.
Subsequently x/y/z is deleted on branch.
The error is generated when running:
    svn merge ^/branches/mybranch
on a trunk working copy.

Sounds to me like a bug, svn tries to update the mergeinfo on the deleted path.
Is that so ?

Using SVN clietn 1.8.1

PS: Please CC me, I'm not on the list.