The svnbook says, "Keep this in mind when defining file externals. If you need the external to refer to a certain revision of a file you will not be able to modify the external. If you want to be able to modify the external, you cannot specify a revision other than the HEAD revision, which is implied if no revision is specified." But in reality this is possible! During a 'svn up' it is possible to postpone a conflict and later resolve and commit the modification on a file externals with certain revision. Reproduction recipe is on the way.
Original issue reported by stylesen