diff --git a/mercurial/bundlerepo.py b/mercurial/bundlerepo.py
--- a/mercurial/bundlerepo.py
+++ b/mercurial/bundlerepo.py
@@ -204,6 +204,8 @@ class bundlemanifest(bundlerevlog, manif
         dirlogstarts=None,
         dir=b'',
     ):
+        # XXX manifestrevlog is not actually a revlog , so mixing it with
+        # bundlerevlog is not a good idea.
         manifest.manifestrevlog.__init__(self, nodeconstants, opener, tree=dir)
         bundlerevlog.__init__(
             self,
diff --git a/mercurial/unionrepo.py b/mercurial/unionrepo.py
--- a/mercurial/unionrepo.py
+++ b/mercurial/unionrepo.py
@@ -205,6 +205,8 @@ class unionchangelog(unionrevlog, change
 
 class unionmanifest(unionrevlog, manifest.manifestrevlog):
     def __init__(self, nodeconstants, opener, opener2, linkmapper):
+        # XXX manifestrevlog is not actually a revlog , so mixing it with
+        # bundlerevlog is not a good idea.
         manifest.manifestrevlog.__init__(self, nodeconstants, opener)
         manifest2 = manifest.manifestrevlog(nodeconstants, opener2)
         unionrevlog.__init__(