Main.cs

using System;
using System.Collections.Generic;

namespace ObjectClonerCrashDemo
{
	class MainClass
	{
		public static void Main (string[] args)
		{
			//This runs fine inside MonoDevelop/VisualStudio 2012
			Lib.TestRunner.Run();
			
			Console.WriteLine("Done");

			Console.Read();
		}
	}
}

Notes

As you can see, it crashes weirdly at line 24 in ObjectExtensions with a NullReferenceException, which is weird since there is a null check on this object on the previous line. 
Doing anything to the object (ie: trying to log it) just moves the error inside String's constructor or String.Format.
Surrounding the line with a try/catch and adding a breakpoint in the catch crashes Unity as soon as the breakpoint is hit, but you can step-in up to this point.

You can download a ZIP file containing the VS2012 solution and Unity demo project here if you want to take a look : https://mega.co.nz/#!0AdlibqB!X5M6T07z58rqLHIEpi7JSk5nvAJICocV_DiEdcuhZy0

TestScript.cs

using UnityEngine;
using System.Collections;

public class TestScript : MonoBehaviour
{
	void Start ()
	{
		//Crash happens inside here in Unity
		Lib.TestRunner.Run();
	}
}

ToClone.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;

namespace Lib
{
  public static class TestRunner
	{
		public static void Run()
		{
			ToClone temp = new ToClone();
			temp.shouldWork = 1234;
			temp.willCrash = new List<object>() { new object(), new object() };
			
			var clone = ObjectExtensions.Copy(temp);
			
			Debug.Assert(clone.shouldWork == temp.shouldWork);
			Debug.Assert(clone.willCrash.Count == temp.willCrash.Count);
		}
	}

	public class ToClone
	{
		public int shouldWork;
		public List<object> willCrash;
	}
}

UnityStackTrace

NullReferenceException: Object reference not set to an instance of an object
System.ObjectExtensions.InternalCopy (System.Object originalObject, IDictionary`2 visited) (at c:/Users/fDeschenes/Desktop/ObjectClonerCrashDemo/Lib/ObjectExtensions.cs:24)
System.ObjectExtensions+<>c__DisplayClass3.<InternalCopy>b__0 (System.Array array, System.Int32[] indices) (at c:/Users/fDeschenes/Desktop/ObjectClonerCrashDemo/Lib/ObjectExtensions.cs:34)
System.ArrayExtensions.ArrayExtensions.ForEach (System.Array array, System.Action`2 action) (at c:/Users/fDeschenes/Desktop/ObjectClonerCrashDemo/Lib/ObjectExtensions.cs:91)
System.ObjectExtensions.InternalCopy (System.Object originalObject, IDictionary`2 visited) (at c:/Users/fDeschenes/Desktop/ObjectClonerCrashDemo/Lib/ObjectExtensions.cs:34)
System.ObjectExtensions.CopyFields (System.Object originalObject, IDictionary`2 visited, System.Object cloneObject, System.Type typeToReflect, BindingFlags bindingFlags, System.Func`2 filter) (at c:/Users/fDeschenes/Desktop/ObjectClonerCrashDemo/Lib/ObjectExtensions.cs:60)
System.ObjectExtensions.InternalCopy (System.Object originalObject, IDictionary`2 visited) (at c:/Users/fDeschenes/Desktop/ObjectClonerCrashDemo/Lib/ObjectExtensions.cs:39)
System.ObjectExtensions.CopyFields (System.Object originalObject, IDictionary`2 visited, System.Object cloneObject, System.Type typeToReflect, BindingFlags bindingFlags, System.Func`2 filter) (at c:/Users/fDeschenes/Desktop/ObjectClonerCrashDemo/Lib/ObjectExtensions.cs:60)
System.ObjectExtensions.InternalCopy (System.Object originalObject, IDictionary`2 visited) (at c:/Users/fDeschenes/Desktop/ObjectClonerCrashDemo/Lib/ObjectExtensions.cs:39)
System.ObjectExtensions.Copy (System.Object originalObject) (at c:/Users/fDeschenes/Desktop/ObjectClonerCrashDemo/Lib/ObjectExtensions.cs:19)
System.ObjectExtensions.Copy[ToClone] (Lib.ToClone original) (at c:/Users/fDeschenes/Desktop/ObjectClonerCrashDemo/Lib/ObjectExtensions.cs:66)
Lib.TestRunner.Run () (at c:/Users/fDeschenes/Desktop/ObjectClonerCrashDemo/Lib/TestRunner.cs:14)
TestScript.Start () (at Assets/Scripts/TestScript.cs:9)