Inspector view changed! Looks totally different. Apply, Revert, ... prefab buttons gone. Help!

My inspector view suddenly changed. Now I cannot apply prefabs! How can I fix this? I tried reverting the window to “factory settings” but that didn’t help anything.

Also, “Layer”, “Name” etc changed into fields as can be seen in the image. I’ve been using Unity for a few years now and this never happened before. :S

For googlers :
this [CustomEditor(typeof(GameObject))] is the cause.