In any project, go to Player Settings and change Gamma to Linear in the Color Space dropdown.

InvalidOperationException: Operation is not valid due to the current state of the object
System.Collections.Stack.Peek () (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.Collections/Stack.cs:321)
UnityEngine.GUILayoutUtility.EndLayoutGroup () (at C:/buildslave/unity/build/artifacts/generated/common/runtime/GUILayoutUtility.gen.cs:223)
UnityEngine.GUILayout.EndVertical () (at C:/buildslave/unity/build/artifacts/generated/common/runtime/GUILayout.gen.cs:266)
UnityEditor.EditorGUILayout.EndVertical () (at C:/buildslave/unity/build/Editor/Mono/EditorGUI.cs:6578)
UnityEditor.InspectorWindow.DrawEditor (UnityEditor.Editor editor, Int32 editorIndex, Boolean forceDirty, System.Boolean& showImportedObjectBarNext, UnityEngine.Rect& importedObjectBarRect, Boolean eyeDropperDirty) (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1166)
UnityEditor.InspectorWindow.DrawEditors (UnityEditor.Editor[] editors) (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:960)
UnityEditor.InspectorWindow.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:342)
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)
UnityEditor.PopupCallbackInfo:SetEnumValueDelegate(Object, String[], Int32)

Regression introduced in Unity 5.0 RC3. RC2 and earlier builds don't have the error.