If you are editing a Prefab in Prefab Mode, add another Prefab to the hierarchy and then back out and discard changes, an assertion failure will take place.

Assertion failed on expression: 'prefabInstanceFileID != 0'
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

Assertion failed on expression: 'fidA != 0'
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

1. Install Unity
2. Open the attached project
3. Find PrefabA in Assets and open in Prefab Mode
4. Make sure Autosave is OFF
5. Add PrefabB to PrefabA
6. Exit Prefab Mode and discard changes

Result:
Assertion Failures occur.

Expected Result:
No errors should appear in the Console.