A meteorological observation at a given place can be inaccurate for a variety of reasons, such as a hardware defect.
[1] Weather observation quality control systems verify probability, history, and trends.
Another basic quality control check is to have the data compared to preset geographic extremes,[3] perhaps combined with diurnal variations.
As already described the program checks whether the observation is within predetermined limits that are set according to whether they can physically exist or not.
The third part is internal checking, which compares the observation to previous ones and sees whether it makes sense or not.