I'm using beta 19 on Windows and still quite constantly get the error "Asset 'C:/Program Files (x86)/Unity/Editor/Data/UnityExtensions/Unity/GUISystem/4.6.0/UnityEngine.UI.dll' is in timestamps but is not known in assetdatabase..." despite the patch notes saying it should be fixed. What usually happens is I try to quit Unity by pressing the close button (X) and nothing seems to happen. I press it again after a while and Unity closes. I start the project again and it's messed up, gives me this error. The uGUI elements already added to a scene still work, but I can't add any new uGUI components. I have to delete the library and reimport everything (which takes a long time with our project).