diff --git a/IPython/core/events.py b/IPython/core/events.py
index 798ef01..bfd09fe 100644
--- a/IPython/core/events.py
+++ b/IPython/core/events.py
@@ -69,7 +69,7 @@ class EventManager(object):
         Any additional arguments are passed to all callbacks registered for this
         event. Exceptions raised by callbacks are caught, and a message printed.
         """
-        for func in self.callbacks[event]:
+        for func in self.callbacks[event][:]:
             try:
                 func(*args, **kwargs)
             except Exception:
diff --git a/IPython/core/tests/test_events.py b/IPython/core/tests/test_events.py
index 8e8402c..3053a70 100644
--- a/IPython/core/tests/test_events.py
+++ b/IPython/core/tests/test_events.py
@@ -30,3 +30,25 @@ class CallbackTests(unittest.TestCase):
         self.em.register('ping_received', cb)
         with tt.AssertPrints("Error in callback"):
             self.em.trigger('ping_received')
+
+    def test_unregister_during_callback(self):
+        invoked = [False] * 3
+        
+        def func1(*_):
+            invoked[0] = True
+            self.em.unregister('ping_received', func1)
+            self.em.register('ping_received', func3)
+
+        def func2(*_):
+            invoked[1] = True
+            self.em.unregister('ping_received', func2)
+
+        def func3(*_):
+            invoked[2] = True
+            
+        self.em.register('ping_received', func1)
+        self.em.register('ping_received', func2)
+
+        self.em.trigger('ping_received')
+        self.assertEqual([True, True, False], invoked)
+        self.assertEqual([func3], self.em.callbacks['ping_received'])
diff --git a/docs/source/whatsnew/pr/incompat-event-triggering.rst b/docs/source/whatsnew/pr/incompat-event-triggering.rst
new file mode 100644
index 0000000..432af68
--- /dev/null
+++ b/docs/source/whatsnew/pr/incompat-event-triggering.rst
@@ -0,0 +1,7 @@
+Update IPython event triggering to ensure callback registration and
+unregistration only affects the set of callbacks the *next* time that event is
+triggered. See :ghissue:`9447` and :ghpull:`9453`.
+
+This is a change to the existing semantics, wherein one callback registering a
+second callback when triggered for an event would previously be invoked for
+that same event.