Steps to reproduce:

1. Open Unity;
2. Open attached project and 'test' scene;
3. Select Terrain component in the Hierarchy;
4. Go to TerrainSettings tab in Terrain Component's Inspector settings;
5. Enter Play Mode;
6. Toggle some 'Base Terrain' and 'Tree and Detail' objects settings - change values or just drag sliders;
7. Observe the console.

Actual result:

InvalidOperationException: This cannot be used during play mode.
UnityEditor.SceneManagement.EditorSceneManager.MarkSceneDirty (Scene scene) (at /Users/builduser/buildslave/unity/build/artifacts/generated/common/editor/EditorSceneManagerBindings.gen.cs:156)
UnityEditor.TerrainInspector.ShowSettings () (at /Users/builduser/buildslave/unity/build/Editor/Mono/TerrainEditor/TerrainInspector.cs:1118)
UnityEditor.TerrainInspector.OnInspectorGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/TerrainEditor/TerrainInspector.cs:1475)
UnityEditor.InspectorWindow.DrawEditor (UnityEditor.Editor editor, Int32 editorIndex, Boolean rebuildOptimizedGUIBlock, System.Boolean& showImportedObjectBarNext, UnityEngine.Rect& importedObjectBarRect) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1235)
UnityEditor.DockArea:OnGUI()

appears in the console. See attached video and actual.png screenshot.

Regression introduced in Unity 5.4.

Expected result:
This assert should not be displayed, since it's unrelated to Terrain component and toggled settings are still applied to Terrain component.

Notes:
- Reproducible on both OSX 10.11.4 and Windows 10;
- Reproducible in 5.4.0b20, 5.5.0a1;
- Not reproducible in 5.3.5p1;
- Not reproducible for toggling 'Wind Settings for Grass', 'Resolution' or 'Heightmap' sections of Terrain component;
- Not reproducible for toggling those settings, while not being in Play mode.