[QUESTION] Here are Common Mistakes When Working with Enumerables. Can you see it?

The following code contains some of the most common mistakes I have been seeing when reviewing code that deals with enumerable objects.

public static IEnumerable<T> ElementsFromTheFirstWhichAreAlsoInTheSecond<T>(
    IEnumerable<T> enumerable1,
    IEnumerable<T> enumerable2
)
{
    var result = new List<T>();

    enumerable1.ToList().ForEach(a =>
    {
        if (enumerable2.Contains(a))
        {
            result.Add(a);
        }
    });

    return result;
}

Can you see what is wrong? Share in the comments.