Reliability has to do with how consistent the measures in an experiment are, this has to do with how reproduceable those measures are under the same circumstance. While validity has to do with the correctness or accuracy of measures in an experiment.
It is possible for a measurement to be unreliable and yet valid because even reliable (consistent measures) may not always necessarily be correct. Experiments involving living things sometimes deviate from normal measures due to several factors that cannot be explained but that does not mean that the results are invalid.
