MGDIS is a French software development company which successfully migrated her monolith application towards a microservices architecture. This migration was due to a major strategic and technical change and needed an investment of 17 300 person.days over three years. While some results were initially targeted, others were not expected at all. However, with hindsight following an afterward in-depth analysis, they eventually proved to be unexpected best-practices that should be adopted for successful microservices adoption. These lessons learned are outlined following four crucial aspects: functional approach, norms and standards, microservices granularity and their semantics, and finally technical and integration outcomes.