What is the New Enemy Problem?

The "New Enemy Problem" is an issue that can appear in systems where resources and permissions are distributed amongst replicated data sources, and therefore, not bounded by time in how "out of date" they can get.

Imagine a system where there are two databases, one in the United States and one in Europe, with updates being replicated from one database to the other, as changes are made. Lets now say a user ("Alice") in the United States removes another user’s ("Bob") permission on a document, and then immediately afterwards updates the contents of the document with information that Bob cannot know about. If the updates to the document reach the European replica of the database before the permission updates, then Bob could see the new information, which is caused by Bob’s permission change having arrived out-of-order.