A student uses a spectrophotometer to determine the molar concentration of \(\text{Cu}^{2+}\text{(aq)}\) in an unknown solution. The student first prepares five standard solutions of known concentration and constructs an accurate linear calibration plot of absorbance versus concentration at \(\lambda_{\text{max}} = 635 \text{ nm}\). The spectrophotometer is calibrated such that absorbance is defined as \(A = -\log_{10}(I/I_0)\), where \(I_0\) is the light intensity transmitted through a blank sample of distilled water and \(I\) is the light intensity transmitted through the sample. Which of the following procedural errors would result in an experimental absorbance reading for the unknown solution that leads to an underestimate of its concentration?