Common mistakes to avoid when designing an irrigation system for golf courses and sports fields include undersizing or oversizing the system, inadequate pipe sizing, poor sprinkler selection, and insufficient system automation. Undersizing the system can lead to inadequate water coverage, while oversizing can result in wasted water and energy. Inadequate pipe sizing can cause excessive pressure loss, reducing system performance. Poor sprinkler selection can lead to inconsistent water application, while insufficient system automation can make it difficult to optimize water usage and respond to system faults or weather events.