Masses and volumes are critical in any chemical reaction. An extra drop of a certain reactant can lead to a disaster.
Also, the combination of certain compounds or elements together can give drastic outputs.
Therefore, you should be very accurate in your measurements to the masses and volumes of the compounds that you'll use.
Based on this, before measuring a liquid volume in a graduated cylinder, you need to be absolutely sure that the cylinder is completely dry and does not contain any traces of any other liquid.