I got this one wrong on my first try because the requirements were ambiguous (to me, at least). Problem 8 reads:

Find the greatest product of five consecutive digits in the 1000-digit number.

73167176531330624919225119674426574742355349194934 96983520312774506326239578318016984801869478851843 85861560789112949495459501737958331952853208805511 12540698747158523863050715693290963295227443043557 66896648950445244523161731856403098711121722383113 62229893423380308135336276614282806444486645238749 30358907296290491560440772390713810515859307960866 70172427121883998797908792274921901699720888093776 65727333001053367881220235421809751254540594752243 52584907711670556013604839586446706324415722155397 53697817977846174064955149290862569321978468622482 83972241375657056057490261407972968652414535100474 82166370484403199890008895243450658541227588666881 16427171479924442928230863465674813919123162824586 17866458359124566529476545682848912883142607690042 24219022671055626321111109370544217506941658960408 07198403850962455444362981230987879927244284909188 84580156166097919133875499200524063689912560717606 05886116467109405077541002256983155200055935729725 71636269561882670428252483600823257530420752963450

I took this to mean the product of five digits that are consecutive in the natural sequence of numbers, i.e. 12345, 23456, 34567, 45678, or 56789. The largest of those that occurs in the 1,000-digit number is 720, or 23456. When Project Euler told me I had gotten this one wrong, some quick googling revealed that they meant the product of five digits that occur consecutively in the 1,000-digit number. Oops. No wonder it seemed too easy.

Then I had the genius idea that by "in the 1,000-digit number," they meant it had to appear in the 1,000-digit number. Well, the largest product of five consecutive digits of the big scary number that itself appears in the number is 9072. Euler told me that was wrong, too, so I went back to google, actually looked at a couple solutions, and noticed that no one had a method to see what product appeared in the number, so I tried feeding Euler the largest product of five consecutive digits in the number, and, lo and behold, it was right. After misinterpreting the directions twice, it turned out to be a very simple problem.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Problem8
{
    class Program
    {
        static void Main(string[] args)
        {
            string bigScaryNumber = "73167176531330624919225119674426574742355349194934" +
                                    "96983520312774506326239578318016984801869478851843" +
                                    "85861560789112949495459501737958331952853208805511" +
                                    "12540698747158523863050715693290963295227443043557" +
                                    "66896648950445244523161731856403098711121722383113" +
                                    "62229893423380308135336276614282806444486645238749" +
                                    "30358907296290491560440772390713810515859307960866" +
                                    "70172427121883998797908792274921901699720888093776" +
                                    "65727333001053367881220235421809751254540594752243" +
                                    "52584907711670556013604839586446706324415722155397" +
                                    "53697817977846174064955149290862569321978468622482" +
                                    "83972241375657056057490261407972968652414535100474" +
                                    "82166370484403199890008895243450658541227588666881" +
                                    "16427171479924442928230863465674813919123162824586" +
                                    "17866458359124566529476545682848912883142607690042" +
                                    "24219022671055626321111109370544217506941658960408" +
                                    "07198403850962455444362981230987879927244284909188" +
                                    "84580156166097919133875499200524063689912560717606" +
                                    "05886116467109405077541002256983155200055935729725" +
                                    "71636269561882670428252483600823257530420752963450";
            int[] consecutiveDigitProducts = new int[bigScaryNumber.Length - 5];

            for (int i = 0; i < consecutiveDigitProducts.Length; i++)
            {
                consecutiveDigitProducts[i] = (int)(Convert.ToInt32(bigScaryNumber[i].ToString()) *
                                                    Convert.ToInt32(bigScaryNumber[i + 1].ToString()) *
                                                    Convert.ToInt32(bigScaryNumber[i + 2].ToString()) *
                                                    Convert.ToInt32(bigScaryNumber[i + 3].ToString()) *
                                                    Convert.ToInt32(bigScaryNumber[i + 4].ToString()));
            }

            Console.WriteLine("The largest product of five consecutive numbers " +
                              "to appear in the given 1,000-digit number is " + consecutiveDigitProducts.Max());
        }
    }
}