When calibrating glass thermometers, errors may arise from multiple sources. Understanding these sources and taking appropriate corrective measures is key to improving calibration accuracy.

1. Manufacturing Errors
   Thermometers may have inaccurate scales or uneven liquid filling during manufacturing. Such errors usually need to be corrected by comparison with a standard thermometer.

2. Wear and Tear
   Long-term use may cause glass deformation or liquid evaporation, introducing errors. Regular inspection and replacement of aging thermometers are effective ways to reduce such errors.

3. Environmental Factors
   Temperature fluctuations, humidity changes, and light interference may affect calibration results. Controlling the calibration environment can minimize these errors.

4. Improper Operation
   For example, inserting the thermometer non-vertically into the calibration medium or reading at an angle can cause errors. Operators should strictly follow calibration procedures.