Full Idea
Reliabilism is open to the counterexample that a belief may be the result of some generally reliable process (a pressure gauge) which was in fact malfunctioning on this occasion, when we would be reluctant to attribute knowledge to the subject.
Gist of Idea
A true belief might be based on a generally reliable process that failed on this occasion
Source
Simon Blackburn (Oxford Dictionary of Philosophy [1994], p.327)
Book Reference
Blackburn,Simon: 'Oxford Dictionary of Philosophy' [OUP 1996], p.327
A Reaction
Russell's stopped clock that tells the right time twice a day. A good objection. Coming from a reliable source is very good criterion for good justification, but it needs critical assessment.