diff --git a/lib/unexpectedMitm.js b/lib/unexpectedMitm.js
index <HASH>..<HASH> 100644
--- a/lib/unexpectedMitm.js
+++ b/lib/unexpectedMitm.js
@@ -553,6 +553,11 @@ module.exports = {
                                     that.errorMode = 'nested';
                                     callback(err);
                                 }).then(function () {
+                                    // ensure the callback is called once in the error case
+                                    if (callbackCalled) {
+                                        return;
+                                    }
+
                                     that.errorMode = 'nested';
                                     callback(null, mockResponse);
                                 });
Protect a callback from firing twice in the error case.