Are you (semi-)familiar with fflib but sometimes still feeling challenged to write constructive test methods, without copy-pasting everything? This post is more like an extensive guide, showing example code for reference, but also deep-diving into the why and how of certain lines of code.
Are you still requiring Outbound Messages for your implementation? Be aware of the Acknowledgement being responded and keep track of NotificationId to prevent a message overload.
Did you know Salesforce has limited the status codes which can be returned in a HTTP Response? This article lists the missing ones and elaborates on how to resolve this.