Background
Subversion is a version control system. It can run as either it’s own server (svnserve), or as an Apache module (mod_dav_svn.so).
When using the mod_dav_svn module for Apache, and doing an svn copy operation on the repository itself can fail if the VirtualHost configuration for subversion is not correct. Put simply, if Apache itself and mod_dav_svn are serving content from the same path, then conflicts can occur. Apache can get confused if it attempts to serve a physical file instead of routing the request through mod_dav_svn.
“svn copy …” operations will fail, while “svn update …”, “svn commit …”, and “svn checkout …” operations work fine.
Detail of the problem, diagnosing the problem, and the fix are below.