http://docs.python.org/library/subprocess.html#exceptions documentation is wrong at least for the case when shell=True on Linux. An attempt to execute a non-existent file with:

  process = subprocess.Popen("sdfsdf", shell=True, stdout=subprocess.PIPE, stderr=errpipe)
  out, err = process.communicate()
  print out

Results in a message "/bin/sh: sfs: command not found" with no exceptions. Other statements may be invalid too.