Steps to reproduce:
1. Create a New Project
2. Select Main Camera in Hierarchy
3. Set Viewport Rect H to 0.0001 or lower in the Camera component

Result: Errors are shown in the Console.

Reproduced with: 5.6.0b6, 5.5.1p2, 5.4.3p3, 5.3.7p2.

A list of Errors shown in the Console:
Screen position out of view frustum (screen pos 0.000000, 0.000000, 1000.000000) (Camera rect 0 0 860 0)
UnityEditor.DockArea:OnGUI()
Screen position out of view frustum (screen pos 0.000000, 0.420000, 1000.000000) (Camera rect 0 0 860 0)
UnityEditor.DockArea:OnGUI()
Screen position out of view frustum (screen pos 860.000000, 0.420000, 1000.000000) (Camera rect 0 0 860 0)
UnityEditor.DockArea:OnGUI()
Screen position out of view frustum (screen pos 860.000000, 0.000000, 1000.000000) (Camera rect 0 0 860 0)
UnityEditor.DockArea:OnGUI()
Screen position out of view frustum (screen pos 0.000000, 0.000000, 0.300000) (Camera rect 0 0 860 0)
UnityEditor.DockArea:OnGUI()
Screen position out of view frustum (screen pos 0.000000, 0.420000, 0.300000) (Camera rect 0 0 860 0)
UnityEditor.DockArea:OnGUI()
Screen position out of view frustum (screen pos 860.000000, 0.420000, 0.300000) (Camera rect 0 0 860 0)
UnityEditor.DockArea:OnGUI()
Screen position out of view frustum (screen pos 860.000000, 0.000000, 0.300000) (Camera rect 0 0 860 0)
UnityEditor.DockArea:OnGUI()