diff --git a/hgext/git/__init__.py b/hgext/git/__init__.py
--- a/hgext/git/__init__.py
+++ b/hgext/git/__init__.py
@@ -16,6 +16,7 @@
     extensions,
     localrepo,
     pycompat,
+    registrar,
     scmutil,
     store,
     util,
@@ -28,6 +29,13 @@
     index,
 )
 
+configtable = {}
+configitem = registrar.configitem(configtable)
+# git.log-index-cache-miss: internal knob for testing
+configitem(
+    b"git", b"log-index-cache-miss", default=False,
+)
+
 # TODO: extract an interface for this in core
 class gitstore(object):  # store.basicstore):
     def __init__(self, path, vfstype):
@@ -41,13 +49,14 @@
             os.path.normpath(os.path.join(path, b'..', b'.git'))
         )
         self._progress_factory = lambda *args, **kwargs: None
+        self._logfn = lambda x: None
 
     @util.propertycache
     def _db(self):
         # We lazy-create the database because we want to thread a
         # progress callback down to the indexing process if it's
         # required, and we don't have a ui handle in makestore().
-        return index.get_index(self.git, self._progress_factory)
+        return index.get_index(self.git, self._logfn, self._progress_factory)
 
     def join(self, f):
         """Fake store.join method for git repositories.
@@ -276,6 +285,8 @@
     if repo.local() and isinstance(repo.store, gitstore):
         orig = repo.__class__
         repo.store._progress_factory = repo.ui.makeprogress
+        if ui.configbool(b'git', b'log-index-cache-miss'):
+            repo.store._logfn = repo.ui.warn
 
         class gitlocalrepo(orig):
             def _makedirstate(self):
diff --git a/hgext/git/gitlog.py b/hgext/git/gitlog.py
--- a/hgext/git/gitlog.py
+++ b/hgext/git/gitlog.py
@@ -399,7 +399,7 @@
             'refs/hg/internal/latest-commit', oid, force=True
         )
         # Reindex now to pick up changes. We omit the progress
-        # callback because this will be very quick.
+        # and log callbacks because this will be very quick.
         index._index_repo(self.gitrepo, self._db)
         return oid.raw
 
diff --git a/hgext/git/index.py b/hgext/git/index.py
--- a/hgext/git/index.py
+++ b/hgext/git/index.py
@@ -216,7 +216,12 @@
     db.commit()
 
 
-def _index_repo(gitrepo, db, progress_factory=lambda *args, **kwargs: None):
+def _index_repo(
+    gitrepo,
+    db,
+    logfn=lambda x: None,
+    progress_factory=lambda *args, **kwargs: None,
+):
     # Identify all references so we can tell the walker to visit all of them.
     all_refs = gitrepo.listall_references()
     possible_heads = set()
@@ -253,6 +258,7 @@
     cur_cache_heads = {h.hex for h in possible_heads}
     if cur_cache_heads == cache_heads:
         return
+    logfn(b'heads mismatch, rebuilding dagcache\n')
     for start in possible_heads:
         if walker is None:
             walker = gitrepo.walk(start, _OUR_ORDER)
@@ -339,7 +345,9 @@
         prog.complete()
 
 
-def get_index(gitrepo, progress_factory=lambda *args, **kwargs: None):
+def get_index(
+    gitrepo, logfn=lambda x: None, progress_factory=lambda *args, **kwargs: None
+):
     cachepath = os.path.join(
         pycompat.fsencode(gitrepo.path), b'..', b'.hg', b'cache'
     )
@@ -349,5 +357,5 @@
     db = _createdb(dbpath)
     # TODO check against gitrepo heads before doing a full index
     # TODO thread a ui.progress call into this layer
-    _index_repo(gitrepo, db, progress_factory)
+    _index_repo(gitrepo, db, logfn, progress_factory)
     return db
diff --git a/tests/test-git-interop.t b/tests/test-git-interop.t
--- a/tests/test-git-interop.t
+++ b/tests/test-git-interop.t
@@ -36,8 +36,12 @@
   $ cd ..
 
 Now globally enable extension for the rest of the test:
-  $ echo "[extensions]" >> $HGRCPATH
-  > echo "git=" >> $HGRCPATH
+  $ cat <<EOF >> $HGRCPATH
+  > [extensions]
+  > git=
+  > [git]
+  > log-index-cache-miss = yes
+  > EOF
 
 Make a new repo with git:
   $ mkdir foo
@@ -68,11 +72,14 @@
 But if you run hg init --git, it works:
   $ hg init --git
   $ hg id --traceback
+  heads mismatch, rebuilding dagcache
   3d9be8deba43 tip master
   $ hg status
+  heads mismatch, rebuilding dagcache
   ? gamma
 Log works too:
   $ hg log
+  heads mismatch, rebuilding dagcache
   changeset:   1:3d9be8deba43
   bookmark:    master
   tag:         tip
@@ -89,7 +96,8 @@
 
 and bookmarks:
   $ hg bookmarks
-   * master                    1:3d9be8deba43
+   * masterheads mismatch, rebuilding dagcache
+                      1:3d9be8deba43
 
 diff even works transparently in both systems:
   $ echo blah >> alpha
@@ -102,6 +110,7 @@
    alpha
   +blah
   $ hg diff --git
+  heads mismatch, rebuilding dagcache
   diff --git a/alpha b/alpha
   --- a/alpha
   +++ b/alpha
@@ -112,12 +121,15 @@
 Remove a file, it shows as such:
   $ rm alpha
   $ hg status
+  heads mismatch, rebuilding dagcache
   ! alpha
   ? gamma
 
 Revert works:
   $ hg revert alpha --traceback
+  heads mismatch, rebuilding dagcache
   $ hg status
+  heads mismatch, rebuilding dagcache
   ? gamma
   $ git status
   On branch master
@@ -130,6 +142,7 @@
 Add shows sanely in both:
   $ hg add gamma
   $ hg status
+  heads mismatch, rebuilding dagcache
   A gamma
   $ hg files
   alpha
@@ -148,7 +161,9 @@
 
 forget does what it should as well:
   $ hg forget gamma
+  heads mismatch, rebuilding dagcache
   $ hg status
+  heads mismatch, rebuilding dagcache
   ? gamma
   $ git status
   On branch master
@@ -165,11 +180,15 @@
 
   $ echo a >> alpha
   $ hg ci -m 'more alpha' --traceback --date '1583522787 18000'
+  heads mismatch, rebuilding dagcache
   $ echo b >> beta
   $ hg ci -m 'more beta'
+  heads mismatch, rebuilding dagcache
   $ echo a >> alpha
   $ hg ci -m 'even more alpha'
+  heads mismatch, rebuilding dagcache
   $ hg log -G alpha
+  heads mismatch, rebuilding dagcache
   @  changeset:   4:6626247b7dc8
   :  bookmark:    master
   :  tag:         tip
@@ -188,6 +207,7 @@
      summary:     Add alpha
   
   $ hg log -G beta
+  heads mismatch, rebuilding dagcache
   o  changeset:   3:d8ee22687733
   :  user:        test <test>
   :  date:        Thu Jan 01 00:00:00 1970 +0000
@@ -200,15 +220,18 @@
   
 
   $ hg log -r "children(3d9be8deba43)" -T"{node|short} {children}\n"
+  heads mismatch, rebuilding dagcache
   a1983dd7fb19 3:d8ee22687733
 
 hg annotate
 
   $ hg annotate alpha
+  heads mismatch, rebuilding dagcache
   0: alpha
   2: a
   4: a
   $ hg annotate beta
+  heads mismatch, rebuilding dagcache
   1: beta
   3: b
 
@@ -219,6 +242,7 @@
   $ mkdir a
   $ echo "This is file mu." > a/mu
   $ hg ci -A -m 'Introduce file a/mu'
+  heads mismatch, rebuilding dagcache
   adding a/mu
 
 Both hg and git agree a/mu is part of the repo
@@ -238,10 +262,12 @@
   On branch master
   nothing to commit, working tree clean
   $ hg status
+  heads mismatch, rebuilding dagcache
 
 
 node|shortest works correctly
   $ hg log -T '{node}\n' | sort
+  heads mismatch, rebuilding dagcache
   3d9be8deba43482be2c81a4cb4be1f10d85fa8bc
   6626247b7dc8f231b183b8a4761c89139baca2ad
   a1983dd7fb19cbd83ad5a1c2fc8bf3d775dea12f
@@ -249,13 +275,16 @@
   c5864c9d16fb3431fe2c175ff84dc6accdbb2c18
   d8ee22687733a1991813560b15128cd9734f4b48
   $ hg log -r ae1ab744f95bfd5b07cf573baef98a778058537b --template "{shortest(node,1)}\n"
+  heads mismatch, rebuilding dagcache
   ae
 
 This coveres changelog.findmissing()
   $ hg merge --preview 3d9be8deba43
+  heads mismatch, rebuilding dagcache
 
 This covers manifest.diff()
   $ hg diff -c 3d9be8deba43
+  heads mismatch, rebuilding dagcache
   diff -r c5864c9d16fb -r 3d9be8deba43 beta
   --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
   +++ b/beta	Mon Jan 01 00:00:11 2007 +0000