  A recent claim by Meyer, Kent and Clifton (MKC), that their models
``nullify'' the Kochen-Specker theorem, has attracted much comment. In this
paper we present a new counter-argument, based on the fact that a classical
measurement reveals, not simply a pre-existing value, but pre-existing
classical information. In the MKC models measurements do not generally reveal
pre-existing classical information. Consequently, the Kochen-Specker theorem is
not nullified. We go on to prove a generalized version of the Kochen-Specker
theorem, applying to non-ideal quantum measurements. The theorem was inspired
by the work of Simon et al and Larsson (SBZL). However, there is a loophole in
SBZL's argument, which means that their result is invalid (operational
non-contextuality is not inconsistent with the empirical predictions of quantum
mechanics). Our treatment resolves this difficulty. We conclude by discussing
the question, whether the MKC models can reproduce the empirical predictions of
quantum mechanics.
