Some common mistakes to avoid when developing a plumbing website include poor user experience design, lack of updated information and content, slow loading times and broken links. These issues can negatively impact the user experience and hurt your website's overall effectiveness in attracting customers. It is essential to regularly review and update your website to ensure all information is accurate and functioning correctly. You should also continually monitor website performance, address any technical issues promptly and make necessary adjustments to improve the user experience continuously.