I have not been able to produce the error.  Here is my code:

------------------------------------------------------------------
   public class ACComboBox : System.Windows.Forms.ComboBox {
       public ACComboBox() : base() {
       }

       protected override void
OnKeyPress(System.Windows.Forms.KeyPressEventArgs e) {
           DroppedDown = true;

           if (char.IsControl(e.KeyChar))
               return;

           string ToFind = Text.Substring(0, SelectionStart) +
e.KeyChar; int Index = FindStringExact(ToFind);
           if (Index == -1)
               Index = FindString(ToFind);

           if (Index == -1)
               return;

           SelectedIndex = Index;
           SelectionStart = ToFind.Length;
           SelectionLength = Text.Length - SelectionStart;
           e.Handled = true;
           base.OnKeyPress(e);
       }

       protected override void OnLeave(System.EventArgs e) {
           string text = Text;
           int index = FindStringExact(text, -1);
           if (index < 0)
               index = 0;

           SelectedIndex = index;
           base.OnLeave(e);
       }
}
------------------------------------------------------------------

public partial class Form1 : Form
   {
       public Form1()
       {
           InitializeComponent();
       }

       private void Form1_Load(object sender, EventArgs e) {
           ArrayList types = new ArrayList();
           foreach (Type t in
this.GetType().BaseType.Assembly.GetTypes()) {
               types.Add(t.Name);
           }

           types.Sort();

           foreach (string type in types) {
               acComboBox1.Items.Add(type);
           }
       }

       private void button1_Click(object sender, EventArgs e) {
           MessageBox.Show(acComboBox1.SelectedItem.ToString());
       }
   }

--------------------------------------------------------------------------------------

   partial class Form1
   {
       /// <summary>
       /// Required designer variable.
       /// </summary>
       private System.ComponentModel.IContainer components = null;

       /// <summary>
       /// Clean up any resources being used.
       /// </summary>
       /// <param name="disposing">true if managed resources should be
disposed; otherwise, false.</param>
       protected override void Dispose(bool disposing)
       {
           if (disposing && (components != null))
           {
               components.Dispose();
           }
           base.Dispose(disposing);
       }

       #region Windows Form Designer generated code

       /// <summary>
       /// Required method for Designer support - do not modify
       /// the contents of this method with the code editor.
       /// </summary>
       private void InitializeComponent()
       {
           this.button1 = new System.Windows.Forms.Button();
           this.acComboBox1 = new
MSDN_AutoCompleteDropDown.ACComboBox();
           this.textBox1 = new System.Windows.Forms.TextBox();
           this.SuspendLayout();
           //
           // button1
           //
           this.button1.Location = new System.Drawing.Point(458, 64);
           this.button1.Name = "button1";
           this.button1.Size = new System.Drawing.Size(75, 23);
           this.button1.TabIndex = 2;
           this.button1.Text = "button1";
           this.button1.Click += new
System.EventHandler(this.button1_Click);
           //
           // acComboBox1
           //
           this.acComboBox1.Location = new System.Drawing.Point(40,
63);
           this.acComboBox1.Name = "acComboBox1";
           this.acComboBox1.Size = new System.Drawing.Size(412, 24);
           this.acComboBox1.TabIndex = 1;
           //
           // textBox1
           //
           this.textBox1.Location = new System.Drawing.Point(40, 35);
           this.textBox1.Name = "textBox1";
           this.textBox1.Size = new System.Drawing.Size(100, 22);
           this.textBox1.TabIndex = 0;
           //
           // Form1
           //
           this.ClientSize = new System.Drawing.Size(772, 257);
           this.Controls.Add(this.textBox1);
           this.Controls.Add(this.button1);
           this.Controls.Add(this.acComboBox1);
           this.Name = "Form1";
           this.Text = "Form1";
           this.Load += new System.EventHandler(this.Form1_Load);
           this.ResumeLayout(false);
           this.PerformLayout();

       }

       #endregion

       private ACComboBox acComboBox1;
       private System.Windows.Forms.Button button1;
       private System.Windows.Forms.TextBox textBox1;
   }

--
Bryan Phillips
MCSD, MCDBA, MCSE
Blog:  http://bphillips76.spaces.live.com