public partial class RadForm1 : Telerik.WinControls.UI.RadForm
{
    public RadForm1()
    {
        InitializeComponent(); 

        this.textBox1.MouseEnter += Control_Enter;
        this.textBox1.MouseLeave += Control_Leave;

    }

    bool hasLoop = false;
    private void Control_Enter(object sender, EventArgs e)
    {
        radSpellChecker1.AutoSpellCheckControl = sender;
        if (hasLoop)
        {
            return;
        }
        for (int i = 0; i < 1000; i++)
        {
            if (i % 2 == 0)
            {
                radSpellChecker1.AutoSpellCheckControl = null;
            }
            else
            {
                radSpellChecker1.AutoSpellCheckControl = sender;
            }

        }
        hasLoop = true;
    }

    private void Control_Leave(object sender, EventArgs e)
    {
        radSpellChecker1.AutoSpellCheckControl = null;
    }
}