diff --git a/hgext/rebase.py b/hgext/rebase.py
--- a/hgext/rebase.py
+++ b/hgext/rebase.py
@@ -1067,6 +1067,8 @@
             with ui.configoverride(overrides, b'rebase'):
                 return _dorebase(ui, repo, action, opts, inmemory=inmemory)
         except error.InMemoryMergeConflictsError:
+            if 'HGNOREALLYONLYINMEMORYREBASE' in os.environ:
+                raise
             ui.warn(
                 _(
                     b'hit merge conflicts; re-running rebase without in-memory'
diff --git a/tests/test-copies-in-changeset.t b/tests/test-copies-in-changeset.t
--- a/tests/test-copies-in-changeset.t
+++ b/tests/test-copies-in-changeset.t
@@ -356,7 +356,7 @@
   $ hg co -q 0
   $ hg mv a b
   $ hg ci -qm 'rename a to b'
-  $ hg rebase -d 1 --config rebase.experimental.inmemory=yes
+  $ HGNOREALLYONLYINMEMORYREBASE= hg rebase -d 1 --config rebase.experimental.inmemory=yes
   rebasing 2:* "rename a to b" (tip) (glob)
   merging a and b to b
   saved backup bundle to $TESTTMP/rebase-rename/.hg/strip-backup/*-*-rebase.hg (glob)