Introduction Errors occur in the laboratory at any level of the testing process. Recognizing these errors may cause patient distress by delaying diagnosis and management until results are released. This study aims to assess preanalytical errors in the laboratory and suggests methods to prevent them to improve accuracy and efficiency in the hematology department of a tertiary care hospital. The background emphasizes the critical role of preanalytical procedures in ensuring accurate hematological diagnoses and highlights the frequent mistakes that occur during specimen collection, handling, and transportation. Staff training, process standardization, using suitable collecting and transport equipment, putting quality control measures in place, and using automation technologies are some strategies for reducing errors and speeding up turnaround times. To increase diagnostic accuracy, patient care outcomes, and laboratory efficiency, hematology laboratories should address preanalytical mistakes and employ fostering methods. Aims and objectives The objective of this research is to evaluate the frequency and characteristics of preanalytical errors within the hematology laboratory of a tertiary care hospital. The study seeks to gauge the scope of the problem, identify key factors contributing to these errors, such as issues in specimen collection, handling, and transportation, and highlight areas where improvements can be made. Also, it intends to assess how preanalytical errors affect hematological accuracy and turnaround times, with a focus on patient care outcomes, and recommend techniques to reduce preanalytical errors and improve the accuracy of hematological results. Materials and methods This study was conducted at the hematology laboratory of our hospital from January 2023 to June 2024 after getting proper approval from the Institutional Review Board (IRB approval number 294/08/2024/PG/SRB/SMCH). It is a retrospective analytical study, and the study population comprised samples of patients from the emergency, inpatient, and outpatient departments, which included 51,155 complete blood count (CBC) and 5,449 peripheral smear (PS) samples. The study included only test samples sent specifically for hematological analysis, while those sent for cytological, biochemical, or microbiological testing were excluded. The distribution frequency of the samples and the number of rejected samples were analyzed, and the results were then compared and correlated. Results During the study period, a total of 56,604 samples were processed in the hematology laboratory. Of these, 886 samples (1.3%) were rejected due to preanalytical errors. The most frequent error was submitting an insufficient sample (54.17%), while the least frequent was the use of an empty or defective tube (0.4%). In the emergency department, the primary issues were insufficient and clotted samples, while in pediatric cases, errors were mainly due to inadequate or diluted samples. Conclusion Preanalytical errors in hematology laboratories, though often overlooked, can significantly impact diagnostic accuracy and patient care. Our study highlights that a substantial portion of errors arise from inadequate sample collection and handling, particularly in emergency and pediatric cases. Adherence to standard laboratory techniques can dramatically reduce preanalytical errors.