Common mistakes to avoid when conducting a cost-benefit analysis and ROI calculation for an irrigation system include underestimating operational costs, overestimating benefits, and failing to consider all relevant factors, such as maintenance and repair costs. Additionally, using inaccurate or incomplete data, or neglecting to account for variables such as weather patterns, soil type, and crop selection, can lead to flawed ROI calculations. By avoiding these mistakes and conducting a thorough and accurate analysis, you can make informed investment decisions and maximize the ROI of your irrigation system.