Here's 2 script to demonstrate the issue.

Whack this in your scripts directory and attach it to a camera:

using UnityEngine;

[ExecuteInEditMode, RequireComponent(typeof(Camera))]
public class CameraMatrixTest : MonoBehaviour
{
    public void Go()
    {
        var m = camera.projectionMatrix;

        camera.projectionMatrix = m;
    }

    public void Restore()
    {
        camera.ResetAspect();
        camera.ResetProjectionMatrix();
    }
}

And this in your Editor folder:

using UnityEditor;
using UnityEngine;

[CustomEditor(typeof(CameraMatrixTest))]
public class CameraMatrixTestEditor : Editor
{
    private CameraMatrixTest Target { get { return (CameraMatrixTest) target; } }


    public override void OnInspectorGUI()
    {
        base.OnInspectorGUI();

        if(GUILayout.Button("Go!")) Target.Go();
        if(GUILayout.Button("Reset")) Target.Restore();
    }
}

What I get is a flipped camera when assigned the camera's own perspective matrix back to itself! Is it a bug or am I missing a parameter?