=== modified file 'bzrlib/bzrdir.py'
--- bzrlib/bzrdir.py	2009-09-24 04:10:39 +0000
+++ bzrlib/bzrdir.py	2009-09-26 10:05:44 +0000
@@ -1835,6 +1835,10 @@
         try:
             return klass._formats[format_string]
         except KeyError:
+            if len(format_string.splitlines()) > 1:
+                # the hg web frontend will return a full HTML page without a 
+                # 404, even for non-existing files.
+                raise errors.NotBranchError(path=transport.base)
             raise errors.UnknownFormatError(format=format_string, kind='bzrdir')
 
     @classmethod