Many individuals overlook key factors when using a mortgage calculator, such as property taxes, insurance, and maintenance costs, which can significantly impact monthly payments. Another common mistake is failing to account for their credit score, as it affects the interest rate offered. Additionally, some users may underestimate their budget by not considering their long-term financial goals. To ensure accuracy, it's best to consult with a mortgage advisor, who can provide tailored advice based on your specific situation and help you avoid these pitfalls.