Null constructor brain teaser. Simply replace ??? with any type that will cause the unit test to pass. Code used xUnit.

BrainTeaser.cs

[Fact]
public static void NullReturnFromConstructor()
{
    //Any Nullable<T> will work
    Assert.Throws<InvalidOperationException>(() => SudoMakeMeSandwich<???>());
}

private static T SudoMakeMeSandwich<T>() where T : new()
{
    return new T() ?? throw new InvalidOperationException();
}