List broken or pebkac?

Either the Generic List (and HashSet) is broken or ... pebkac

This unworthy test-program outputs the following:

System.Byte[]
6:2 3 4 5 6 7
True        // as expected
System.Byte[]
6:3 4 5 6 7 8
True        // as expected
System.Byte[]
6:3 4 5 6 7 8
False       // oops ... something's very wrong
System.Byte[]
6:3 4 5 6 7 8
False       // oops ... something's very wrong

I just don't get it, REALLY don't get it why the last two ones are "False" :-/

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

public class HashSetTest {

    public static HashSet<byte[]> SomeThing=new HashSet<byte[]>();

    public static void Write(byte[] pebkac) {
        Console.Write("{0}:",pebkac.Length);
        foreach(var i in pebkac) {
            Console.Write("{0} ",i);
        }
        Console.WriteLine();
    }

    public static void Main() {
        byte[] test1= {1,2,3,4,5,6};
        byte[] test2= {2,3,4,5,6,7};
        byte[] test3= {3,4,5,6,7,8};

        List<byte> test4=new List<byte>();
        test4.AddRange(test3);

        byte[] test5=new byte[6];
        var i=0;
        foreach(byte j in test3) {
            test5[i++]=j;
        }

        SomeThing.Add(test1);
        SomeThing.Add(test2);
        SomeThing.Add(test3);

        Console.WriteLine(test2);
        Write(test2);
        Console.WriteLine(SomeThing.Contains(test2));

        Console.WriteLine(test3);
        Write(test3);
        Console.WriteLine(SomeThing.Contains(test3));

        Console.WriteLine(test4.ToArray());
        Write(test4.ToArray());
        Console.WriteLine(SomeThing.Contains(test4.ToArray()));

        Console.WriteLine(test5);
        Write(test5);
        Console.WriteLine(SomeThing.Contains(test5));
    }
}