C#

class Global
{
   public static int a = 20;
}

class C
{
   void Method1()
   {
      Global.a = 30;
      Console.WriteLine(Global.a); // prints 30
      Method2();
      Console.WriteLine(Global.a); // prints 100
   }

   void Method2()
   {
      Global.a = 100;
   }
}

public event EventHandler HandleThis;

void OnHandleThis(EventArgs e)
{
   if (HandleThis != null)
   {
      HandleThis(this, e);
   }
}

void OnHandleThis(EventArgs e)
{
   EventHandler handler = HandleThis;
 
   if (handler != null)
   {
      handler(this, e);
   }
}

Gizz quoted his supplier:

You should only ever add/remove an event handler once in most applications. It is not designed by Microsoft to have event handlers added and removed continuously as a method to controlling these things.

Gizz continued to quote his supplier:

This is because an event handler is usually dealt with in another thread (or how else could it be generated for the application)

and adding/removing notifications to the other thread can easily be the subject of racetrack timing errors.