diff --git a/UPGRADE.rst b/UPGRADE.rst
index 699f04c2c..9f044719a 100644
--- a/UPGRADE.rst
+++ b/UPGRADE.rst
@@ -27,7 +27,7 @@ running:
     # Pull the latest version of the master branch.
     git pull
     # Update the versions of synapse's python dependencies.
-    python synapse/python_dependencies.py | xargs -n1 pip install
+    python synapse/python_dependencies.py | xargs -n1 pip install --upgrade
 
 
 Upgrading to v0.15.0
diff --git a/synapse/handlers/register.py b/synapse/handlers/register.py
index b9b5880d6..dd75c4fec 100644
--- a/synapse/handlers/register.py
+++ b/synapse/handlers/register.py
@@ -53,6 +53,13 @@ class RegistrationHandler(BaseHandler):
                 Codes.INVALID_USERNAME
             )
 
+        if localpart[0] == '_':
+            raise SynapseError(
+                400,
+                "User ID may not begin with _",
+                Codes.INVALID_USERNAME
+            )
+
         user = UserID(localpart, self.hs.hostname)
         user_id = user.to_string()
 
diff --git a/synapse/push/emailpusher.py b/synapse/push/emailpusher.py
index e224b6829..6600c9cd5 100644
--- a/synapse/push/emailpusher.py
+++ b/synapse/push/emailpusher.py
@@ -14,6 +14,7 @@
 # limitations under the License.
 
 from twisted.internet import defer, reactor
+from twisted.internet.error import AlreadyCalled, AlreadyCancelled
 
 import logging
 
@@ -92,7 +93,11 @@ class EmailPusher(object):
 
     def on_stop(self):
         if self.timed_call:
-            self.timed_call.cancel()
+            try:
+                self.timed_call.cancel()
+            except (AlreadyCalled, AlreadyCancelled):
+                pass
+            self.timed_call = None
 
     @defer.inlineCallbacks
     def on_new_notifications(self, min_stream_ordering, max_stream_ordering):
@@ -189,7 +194,10 @@ class EmailPusher(object):
                     soonest_due_at = should_notify_at
 
                 if self.timed_call is not None:
-                    self.timed_call.cancel()
+                    try:
+                        self.timed_call.cancel()
+                    except (AlreadyCalled, AlreadyCancelled):
+                        pass
                     self.timed_call = None
 
         if soonest_due_at is not None:
diff --git a/synapse/push/httppusher.py b/synapse/push/httppusher.py
index 9a7db6122..feedb075e 100644
--- a/synapse/push/httppusher.py
+++ b/synapse/push/httppusher.py
@@ -16,6 +16,7 @@
 from synapse.push import PusherConfigException
 
 from twisted.internet import defer, reactor
+from twisted.internet.error import AlreadyCalled, AlreadyCancelled
 
 import logging
 import push_rule_evaluator
@@ -109,7 +110,11 @@ class HttpPusher(object):
 
     def on_stop(self):
         if self.timed_call:
-            self.timed_call.cancel()
+            try:
+                self.timed_call.cancel()
+            except (AlreadyCalled, AlreadyCancelled):
+                pass
+            self.timed_call = None
 
     @defer.inlineCallbacks
     def _process(self):