Shows possible problem with IEnumerable (http://blog.chaliy.name/post/7274777462/fooken-by-ienumerable).

EnumerableProble.cs

public static void Main (string[] args)
{
    var items = from i in new []{1}
                let x = SideEffect(i)
                select i;

    Somemething(items);
}

public static string SideEffect(int i){
    Console.WriteLine("Side effect");
    return i.ToString();
}

public static void Somemething(IEnumerable<int> input){
    var c = input.Count(); // This will evaluate first time
    foreach(var i in input){ // This will evaluate second time
        var ii = i;
    }
    var d = input.Any(); // This will evaluate third time
}