One thing that I didn't understand when I first started my current job was the difference between internal and external hostnames (and ip addresses and ports). The use of load-balancers for internal as well as external transactions also complicates the issue.

It remains a stumbling block, since, in development and QA environments, the internal and external hostnames are the same, causing many developers to be either ignorant of the difference, or to treat it rather cavalierly.

It's not terribly complicated, but an awareness is required. The internal and external URLs have to be configured separately. Using a single URL configuration is one stumbling block. When sending callback URLs or redirect URLs, for example, one needs to know to send the internal URL when passing it to an internal system, and to send the external URL when passing it to an external system.