To reproduce:
1. Create a script:

using System.Threading;
using UnityEngine;

public class Test : MonoBehaviour
{
private ManualResetEvent m_ResetEvent;
void Start()
{
m_ResetEvent = new ManualResetEvent(false);
var thread = new Thread(WaitPlease);
thread.Start();
}
private void WaitPlease()
{
m_ResetEvent.WaitOne();
Debug.Log("How could I possibly be here?");
}
}

2. Attach the script to the camera
3. Enter/leave play mode - console is clean
4. Attach monodevelop debugger to Unity
5. Enter/leave play mode - notice that the console now has "How could I possibly be here?" - the ManualResetEvent did not work