`beforeSend` was changed at Zepto1.1.1. So `Backbone.sync` is now broken. This patch will fix, maybe. see https://github.com/madrobby/zepto/issues/878

backbone.diff

diff --git a/backbone.js b/backbone.js
index 2171292..fd83e56 100644
--- a/backbone.js
+++ b/backbone.js
@@ -1162,8 +1162,12 @@
       params.type = 'POST';
       if (options.emulateJSON) params.data._method = type;
       var beforeSend = options.beforeSend;
-      options.beforeSend = function(xhr) {
-        xhr.setRequestHeader('X-HTTP-Method-Override', type);
+      options.beforeSend = function(xhr, settings) {
+        if (settings && settings.headers) {
+          settings.headers['X-HTTP-Method-Override'] = type;
+        } else {
+          xhr.setRequestHeader('X-HTTP-Method-Override', type);
+        }
         if (beforeSend) return beforeSend.apply(this, arguments);
       };
     }

This problem has been fixed at this commit in zepto.