Anytime a measurement is outside the normal range of an instrument, it is inaccurate.
You cannot “measure” anything without a reference standard. When a measurement “scale” is used, it is defined by physical limits as well as the availability of standards for comparison.
In the example of absorbance, the scale of “0 – 1” means “no absorbance to complete absorbance”. Physically, you cannot have less than “no absorbance”, and you can have no more than “complete absorbance”.
Thus, if your spectrometer indicates a value either below zero (negative) or greater than 1.0, your instrument is not really properly calibrated. In all cases, the indicated values are at best relative, and usually worthless (invalid) for any quantitative assessments.