It is often the case that the code produced by developers works fine in a development environment but not in production. A lot of the time, that is because the production environment might be very different from other environments and some unforeseen errors may occur. The common mistakes are that in a development environment, services are collocated on the same servers or there isn't the same level of security, so services can communicate with one another in development but not in production. Another issue is that the development environment might not run the same versions of a certain library, and therefore the interface to communicate with them might differ. The development environment may be running a newer version of a service that has new features that production doesn't have yet, or it's simply a question of scale. The dataset used in development isn't as big as that used in production, and scaling issues might crop up once the new code is out in production.