Full Idea
Error, like truth, presupposes judgement. Judgements you make about your conscious states are distinct from those states. This leaves room for error.
Gist of Idea
Error must be possible in introspection, because error is possible in all judgements
Source
John Heil (Philosophy of Mind [1998], Ch.6)
Book Reference
Heil,John: 'Philosophy of Mind' [Routledge 1998], p.210
A Reaction
This sounds very neat. The reply would have to be that a lot of introspection is not judgement, but direct perception of self-evident facts and truths. I agree with Heil.