Anyone able to spot the obvious error here, ignoring the poor code and lack of adherence to standards. My solution produces the correct answer for all of the examples, but fails on the puzzle.

Puzzle: [URL=http://wikisend.com/download/295490/advent.txt]advent.txt[/URL]

class Program { static int[,] Lights = new int[1000, 1000]; static int LightsOn = 0; private static void Main(string[] args) { List<string>Instructions = System.IO.File.ReadAllLines(@"c:\advent.txt").ToList(); var t = 0; var x = 0; for (var i = 0; i <= 999; i++) { for (var j = 0; j <= 999; j++) { Lights[i, j] = 0; } Lights[i, x] = 0; x++; }

        foreach (var Instruc in Instructions)
        {
            DoInstruction(Instruc);
        }

        for (int k = 0; k < Lights.GetLength(0); k++)
            for (int l = 0; l < Lights.GetLength(1); l++)
            {

                var val = Lights[k, l];
                if (val == 1)
                {
                    LightsOn ++;
                }
            }

        Console.WriteLine(LightsOn);
    }

    private static void DoInstruction(string Input)
    {

        var Numbers = ExtractNumbers(Input);
        Console.WriteLine(Numbers[1]);
        var Xstart = Convert.ToInt32(Numbers[1]);
        var Ystart = Convert.ToInt32(Numbers[2]);
        var Xend = Convert.ToInt32(Numbers[3]);
        var Yend = Convert.ToInt32(Numbers[4]);

        var x = Xstart;
        var y = Ystart;

        if (Input.Contains("on"))
        {
            for (var i = x; i <= Xend; i++)
            {
                for (var j = y; j <= Yend; j++)
                {
                    Lights[i, j] = 1;
                }
                Lights[x, i] = 1;
            }
        }
        if (Input.Contains("off"))
        {
            for (var i = x; i <= Xend; i++)
            {
                for (var j = y; j <= Yend; j++)
                {
                    Lights[i, j] = 0;
                }
                Lights[x, i] = 0;
            }
        }
        if (Input.Contains("toggle"))
        {
            for (var i = x; i <= Xend; i++)
            {
                for (var j = y; j <= Yend; j++)
                {
                    if (Lights[i, j] == 1)
                    {
                        Lights[i, j] = 0;
                    }
                    else
                    {
                        Lights[i, j] = 1;
                    }

                }
                if (Lights[x, i] == 1)
                {
                    Lights[x, i] = 0;
                }
                else
                {
                    Lights[x, i] = 1;
                }

            }
        }
    }




    private static string[] ExtractNumbers(string Input)
    {
        string[] Numbers = Regex.Split(Input, @"\D+");
        return Numbers;
    }

}