No, because this deferred may have already fired and is waiting on other work.  In which case that will raise an AlreadyCalledError:

    >>> from twisted.internet import defer
    >>> a = defer.Deferred()
    >>> b = defer.Deferred()
    >>> a.addCallback(lambda _: b)
    <Deferred at 0x101481878>
    >>> a.callback('started')
    >>> a
    <Deferred at 0x101481878 waiting on Deferred at 0x101481a70>
    >>> a.errback(Exception('Timeout me!'))
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/Users/cyli/.virtualenvs/otter/lib/python2.7/site-packages/twisted/internet/defer.py", line 421, in errback
        self._startRunCallbacks(fail)
      File "/Users/cyli/.virtualenvs/otter/lib/python2.7/site-packages/twisted/internet/defer.py", line 481, in _startRunCallbacks
        raise AlreadyCalledError
    twisted.internet.defer.AlreadyCalledError

So we could check to see if it's already been called once maybe, but why since cancellation also implements that functionality?  If this deferred is waiting on the result of another deferred, it forwards the cancellation to that deferred that it's waiting on.

Also, the user has already callbacked or errbacked, canceling it won't raise an error.  There's a special case for it.  We might want to check that case ourselves, and this saves us some code.

It'd be nice if we could just provide a canceller that raises TimedOutError, but we do not create this Deferred.