diff -r dbad4564cd12 Lib/concurrent/futures/_base.py
--- a/Lib/concurrent/futures/_base.py	Mon Jan 27 12:18:49 2014 +0100
+++ b/Lib/concurrent/futures/_base.py	Mon Jan 27 07:43:41 2014 -0500
@@ -225,7 +225,8 @@
 
     finally:
         for f in fs:
-            f._waiters.remove(waiter)
+            with f._condition:
+                f._waiters.remove(waiter)
 
 DoneAndNotDoneFutures = collections.namedtuple(
         'DoneAndNotDoneFutures', 'done not_done')
@@ -272,7 +273,8 @@
 
     waiter.event.wait(timeout)
     for f in fs:
-        f._waiters.remove(waiter)
+        with f._condition:
+            f._waiters.remove(waiter)
 
     done.update(waiter.finished_futures)
     return DoneAndNotDoneFutures(done, set(fs) - done)