Spreadsheet errors are common, with 90% of complex spreadsheets containing errors. People are prone to making mistakes and only moderately good at finding their own errors. Common causes of spreadsheet errors include overconfidence among developers, insufficient time, and lack of development policies. To reduce errors, best practices include preliminary design, testing calculations, and having others review the work. Significant financial errors have occurred when large companies relied on spreadsheets, with billions in incorrect reporting due to minor calculation mistakes.