The post-commit hook for SVN fails to recognize a change in a project that uses svn:externals if the change is in one of the external'ed locations. For example imagine that a project has been set up pointing to parent/trunk which in turn has the following structure:

parent/trunk
  core -> core/trunk
  shared -> shared/trunk

If I then modify a file within core/trunk a build of the project fails to trigger because the only SVN locations it knows about is parent/trunk.