This document discusses 5 common embarrassing mistakes to avoid in email subject lines: 1) Not overwriting placeholder text, 2) Typographical errors, 3) Broken personalization, 4) Improperly inputted or unsupported emojis and special characters, and 5) Awkward truncation. It provides examples of each mistake and recommends double checking spelling, grammar, content, personalization, and previewing subject lines in different email clients to avoid these mistakes.