Index: Lib/subprocess.py
===================================================================
--- Lib/subprocess.py	2009-10-15 23:52:53.000000000 +0200
+++ Lib/subprocess.py	2009-10-15 23:55:54.000000000 +0200
@@ -982,7 +982,10 @@
         def terminate(self):
             """Terminates the process
             """
-            TerminateProcess(self._handle, 1)
+            if self.poll() is None: # child process is still running
+                TerminateProcess(self._handle, 1)
+            else:
+                raise OSError("Child %d has already been terminated." % self.pid)
 
         kill = terminate
 
@@ -1355,7 +1358,10 @@
         def send_signal(self, sig):
             """Send a signal to the process
             """
-            os.kill(self.pid, sig)
+            if self.poll() is None: # child process is still running
+                os.kill(self.pid, sig)
+            else:
+                raise OSError("Child %d has already been terminated." % self.pid)
 
         def terminate(self):
             """Terminate the process with SIGTERM