We use ReSharper with our project, and have previously included a copy of JetBrains.Annotations. See http://answers.unity3d.com/questions/625936/vs-2013-w-resharper-start-update-ongui-is-never-us.html for more info. After upgrading the project to Unity 5, I discovered that UnityEngine.dll has its own copy of the JetBrains.Annotations namespace embedded inside it. Does anyone know if this is intentional and will it be included in all future versions of Unity? If so, I'll remove our copy from the project.