This document discusses 10 common mistakes made when writing T-SQL code. It provides examples and explanations for each mistake, including: using procedural coding instead of set-based methods, overusing user-defined functions and views which can hurt performance, using SELECT * instead of specifying columns, writing non-SARGable queries, relying on "bandaid" fixes instead of proper logic, mixing mismatched data types, improperly handling NULL values, specifying unnecessary or incorrect ordering, and writing single-row triggers that don't account for multiple rows. The document emphasizes the importance of set-based coding, proper data handling, and avoiding shortcuts that can lead to performance and logic issues.