diff --git a/tests/test_sync.py b/tests/test_sync.py
index 52d0882..973bb93 100644
--- a/tests/test_sync.py
+++ b/tests/test_sync.py
@@ -439,6 +439,8 @@ class SyncMachine(RuleBasedStateMachine):
         old_items_b = self._get_items(b)
 
         try:
+            # If one storage is read-only, double-sync because changes don't
+            # get reverted immediately.
             for _ in range(2 if a.read_only or b.read_only else 1):
                 sync(a, b, status,
                      force_delete=force_delete,