The document discusses several common mistakes in software architecture including: not properly scoping a project, not considering all stakeholders, only focusing on functions without considering implementation, forgetting that the system needs to be built, and lack of precision in specifying technical platforms. It also discusses mistakes specific to service-oriented architecture (SOA) like equating SOA with web services and thinking SOA guarantees shortest development times or external system discoverability. The key is to avoid unnecessary complexity, focus on present needs over future-proofing, avoid adding risk without benefit, and not invest in the future at the cost of the present project's success.