I'm attempting a merge with the SVN plugin via the web interface with ikiwiki-3.20100403 and subversion 1.6.11.
The web interface says
Your changes conflict with other changes made to the page. Conflict markers have been inserted into the page content. Reconcile the conflict and commit again to save your changes.
However there are no merge conflict markers in the page. My apache error log says:
[Fri Apr 30 16:43:57 2010] [error] [client 10.64.64.42] svn: Commit failed (details follow):, referer: https://unixwiki.ncl.ac.uk/ikiwiki.cgi [Fri Apr 30 16:43:57 2010] [error] [client 10.64.64.42] svn: Authorization failed, referer: https://unixwiki.ncl.ac.uk/ikiwiki.cgi
Only way for this to be improved would be for the svn plugin to explicitly check the file for conflict markers. I guess it could change the error message then, but the actual behavior of putting the changed file back in the editor so the user can recommit is about right as far as error recovery goes. --Joey