For consideration in response to https://gist.github.com/lucasmeijer/e7bb3a981c2b2887642d

gistfile1.cs

    IEnumerable<string> ErrorsFor3(string[] hayStack, string[] needles, Func<string, string, string> errorMessageGenerator)
    {
        return hayStack.Select(hay => needle.FirstOrDefault(hay.Contains))
            .Where(n => n != null)
            .Select(matchingNeedle => errormessageGenerator(hay, matchingNeedle));
    }