How can an instrument be reliable but not valid?

An instrument can be reliable but not valid when it consistently produces the same results under the same conditions, indicating stability and precision, but does not measure what it is intended to measure. For example, a scale that consistently weighs a person's mass the same way might be reliable, but if it is incorrectly calibrated and always adds five pounds, it is not valid for assessing true weight. Thus, while the results are dependable, they do not accurately reflect the intended construct.