Hello, I'm testing 5.4b11 with my plugin, Advanced Multi-Scene. The plug-in needs to know what kind of state the Editor is in to determine what it should be saving as its state. For instance, it's trying to save off what Scene a particular object belongs to, but it obviously shouldn't do this during the build process (where the scene is considered a temporary scene), and don't want to perform editor-only serialization steps during play mode. The new beta enforces that no APIs can be called during the Serialization callbacks which is breaking this previously working functionality. Specifically, I'm using BuildPipeline.isBuildingPlayer, Application.isPlaying, and AssetDatabase.AssetPathToGUID which was all previously working in all versions of Unity. Previously, you could call these functions and it would sometimes throw an exception if you were caught on the loading thread. However, many things like inspecting in the Editor, and creating SerializableObjects cause the Serialization callbacks to run on the main thread where calling this functionality is perfectly acceptable. Please return the old functionality, as there's no way to work around these issues. There still don't appear to be callbacks when builds start or end and caching states for playing vs. not inside a behaviour is really ugly.