To Reproduce:
Add a RadCarousel - add less than 5 items - you can see that the behavior is incorrect

Workaround:
this.carousel.Items.Add(new RadButtonElement("Text"));
for (int i = 0; i < 4; i++)
{
    this.carousel.Items.Add(new RadButtonElement(){ Visibility = ElementVisibility.Hidden });
}

http://www.telerik.com/forums/using-a-radcarousel-with-a-variable-number-of-items-less-than-5

Workaround:
public Form1()
{
    InitializeComponent();

    this.radCarousel1.CarouselElement.AnimationFinished += CarouselElement_AnimationFinished;
}

void CarouselElement_AnimationFinished(object sender, EventArgs e)
{
    if (this.radCarousel1.Items.Count == 6)
    {
        radCarousel1.CarouselElement.AnimationFinished -= CarouselElement_AnimationFinished;
        this.radCarousel1.SelectedIndex--;
    }
}

Same action with the next button runs smoothly.