This study evaluated the effect of small systematic errors, such as Multi Leave Collimator (MLC), jaw misalignments, and monitor unit errors, on the quality of Intensity Modulated radiotherapy (IMRT) treatment plan delivery. A python code was created to enable the modification of these treatment-planning parameters. Two IMRT QA verification techniques, field-by-field (FBF) and single gantry angle composite (SGAC), were performed to evaluate the original and modified plans using a 2-D ion chamber array detector. The dose distribution measured by the array detector for the FBF and SGAC were compared with the dose distribution calculated by the treatment planning system (TPS). The FBF method was more sensitive than the SGAC for detecting small systematic errors, such as opening and closing the MLC segments, which were evaluated with respect to a gamma-index of 3%/3 mm and 2%/2 mm. The systematic errors involvedin closing the segments of the anterior field by 2 mm and 3 mm showed a significant difference compared with the original field (unmodified): 80.97 ± 1.7% and 41.52 ± 1.9% gamma-index passing rates according to FBF. For SGAC, the magnitude of closing the MLC by 2mm remained unnoticed and resulted in a 96.95 ± 2.61% gamma-index passing rate. The Receiver Operating Characteristic (ROC) test was used to evaluate the clinical impact of the systematic errors. Opening the MLC by 2 mm gave a false negative, but more than 5% of the rectum received 75 Gy, which exceeded the tolerance radiation dose according to Quantitative Analyses of Normal Tissue Effects in the Clinic (QUANTEC).