To reproduce:

Add the following line of code: 
[C#]
this.radSpellChecker1.SpellCheckMode = Telerik.WinControls.UI.SpellCheckMode.AllAtOnce;
[VB.NET]
Me.RadSpellChecker1.SpellCheckMode = Telerik.WinControls.UI.SpellCheckMode.AllAtOnce

When you spell check RadRichTextBox and the spell check form is opened click the change button. You will notice that the word will not changed in the original RadRichTextBox.

this.radSpellChecker1.RegisterControlSpellChecker(typeof(RadRichTextBox), new MyRichTextBoxSpellChecker());
this.radSpellChecker1.SpellCheckMode = SpellCheckMode.AllAtOnce;

Workaround:
class MyRichTextBoxSpellChecker : RadRichTextBoxSpellChecker
{
    public override Telerik.WinControls.RichTextBox.Model.RadDocument GetContentAsDocument()
    {
        return (this.CurrentControl as RadRichTextBox).Document;
    }
}