diff --git a/codes/codes.go b/codes/codes.go
index d9b9d5782e08..02738839dd98 100644
--- a/codes/codes.go
+++ b/codes/codes.go
@@ -132,7 +132,8 @@ const (
 
 	// Unavailable indicates the service is currently unavailable.
 	// This is a most likely a transient condition and may be corrected
-	// by retrying with a backoff.
+	// by retrying with a backoff. Note that it is not always safe to retry
+	// non-idempotent operations.
 	//
 	// See litmus test above for deciding between FailedPrecondition,
 	// Aborted, and Unavailable.