The primary source of error in measurement is the human doing the measurement.
Some measurement devices also influence the item they are measuring. Consider a thermometer. If the thermometer is one temperature and it is inserted into something it is to measure of a different temperature, the thermometer will transfer heat to or from the surrounding material, thereby changing the material's temperature.
When a tool is used for measurement, the engineer should consider how that tool might effect the item being measured and estimate whether that effect is significant or insignificant. Using the example above, a thermometer inserted in a small quantity of a material being measured will have a much greater effect than one inserted into a large quantity of material. To a large extent this determination of whether the tool used will have a significant error is based on the engineer's own experience.