As careful as you may be, sometimes it just happens. Garbage was checked in that needs to be reverted. A client side way of doing this is "reverse merge". I.e. to take the delta that was applied in a particular checkin and reverse merge it on the checked out copy (in a way "subtract" the delta).
svn merge -c -<revision> .The minus in front of the revision is crucial as it will trigger the reverse merge.
I.e. if the checkin that broke something was revision 6791 try
svn up svn merge -c -6791 . svn commit
(c) joachim(et)buechse.ch. This description is in the public domain.