To reproduce: 

Populate the AutoCompleteBox and start a search returning enough results to need the scrollbar. When clicking and dragging the scroll bar to scroll - if the cursor is not positioned directly over the scrollbar at the time you release the mouseclick, the dropdown list is closed an item is added to the selection.

Workaround: 
 public partial class Form1 : Form
    {
        bool isMouseDown = false;

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            AddAutoCompleteItems();
            radAutoCompleteBox1.ListElement.VScrollBar.MouseDown += ListElement_MouseDown;
            radAutoCompleteBox1.ListElement.VScrollBar.MouseUp += ListElement_MouseUp;
            radAutoCompleteBox1.TokenValidating += radAutoCompleteBox1_TokenValidating;
        }

        void radAutoCompleteBox1_TokenValidating(object sender, TokenValidatingEventArgs e)
        {
            e.IsValidToken = !isMouseDown;
            isMouseDown = false;
        }

        void ListElement_MouseUp(object sender, MouseEventArgs e)
        {
            RadListVisualItem el = radAutoCompleteBox1.ListElement.ElementTree.GetElementAtPoint(e.Location) as RadListVisualItem;

            if (el != null)
            {
                return;
            }
            
            isMouseDown = false;
        }

        void ListElement_MouseDown(object sender, MouseEventArgs e)
        {
            isMouseDown = true;
        }

        private void AddAutoCompleteItems()
        {
            RadListDataItemCollection items = this.radAutoCompleteBox1.AutoCompleteItems;

            items.Add(new RadListDataItem("Joe Smith", "joe@fakecompany.com"));
            items.Add(new RadListDataItem("Adam Petersen", "adam@qwerty.com"));
            items.Add(new RadListDataItem("Jack Russel", "jack@russel.nocom"));
            items.Add(new RadListDataItem("Daniel Finger", "daniel.pinger@gmail.com"));
            items.Add(new RadListDataItem("Richard Vail", "rvail@richardvail.com"));
            items.Add(new RadListDataItem("Sebastian Jonnson", "s.jonnson@sjonnson.com"));
            items.Add(new RadListDataItem("Lee Cooper", "lee.cooper@coopercoorp.com"));
            items.Add(new RadListDataItem("Kelvin Clain", "kclain@clainkevin.com"));
            items.Add(new RadListDataItem("Maria Jenson", "mjenson@mariajenson.com"));
            items.Add(new RadListDataItem("Chelsea Maarten", "chelsea@maarten.com"));
            items.Add(new RadListDataItem("Jenson Chew", "jenson.chew@nospam.com"));
            items.Add(new RadListDataItem("Martin Williams", "m.williams@martinandwilliams.com"));
            items.Add(new RadListDataItem("Telerik", "clientservice@telerik.com"));
            items.Add(new RadListDataItem("James Stone", "james.stone@manystones.com"));
            items.Add(new RadListDataItem("Samuel Jackson", "samuel.jackson@nojackson.com"));
        }
    }

    public class MyRadAutoCompleteBox : RadAutoCompleteBox
    {
        protected override RadTextBoxControlElement CreateTextBoxElement()
        {
            return new MyRadAutoCompleteBoxElement();
        }
    }

    public class MyRadAutoCompleteBoxElement : RadAutoCompleteBoxElement
    {
        protected override TextBoxViewElement CreateViewElement()
        {
            return new MyAutoCompleteBoxViewElement();
        }

        protected override Type ThemeEffectiveType
        {
            get
            {
                return typeof(RadAutoCompleteBoxElement);
            }
        }
    }

    public class MyAutoCompleteBoxViewElement : AutoCompleteBoxViewElement
    {
        protected override int InsertTokenizedTextBlocks(int index, string text, bool performInvalidation)
        {
            bool isValid = performInvalidation ? this.OnTokenValidating(text) : true;

            if (!isValid)
            {
                return index;
            }
            return base.InsertTokenizedTextBlocks(index, text, performInvalidation);
        }