I'm trying to implement quicksort in a functional style using C# using linq, and this code randomly works/doesn't work, and I can't figure out why.
Important to mention: When I call this on an array or list, it works fine. But on an unknown-what-it-really-is IEnumerable, it goes insane (loses values or crashes, usually. sometimes works.)
The code:

   public static IEnumerable<T> Quicksort<T>(this IEnumerable<T> source)
        where T : IComparable<T>
    {
        if (!source.Any())
            yield break;
        var pivot = source.First();
        var sortedQuery = source.Skip(1).Where(a => a.CompareTo(source.First()) <= 0).Quicksort()
                          .Concat(new[] { pivot })
                          .Concat(source.Skip(1).Where(a => a.CompareTo(source.First()) > 0).Quicksort());
        foreach (T key in sortedQuery)
            yield return key;
    }

Can you find any faults here that would cause this to fail?