To reproduce:

this.radSpellChecker1.AutoSpellCheckControl = this.radTextBoxControl1;
this.radTextBoxControl1.Text = "this is a test [doc]";

private void radButton1_Click(object sender, EventArgs e)
{
    radSpellChecker1.Check(radTextBoxControl1);
}

The "[doc]" is underlined as misspelled. However, if you click the button, no mistakes will be found.

Workaround:

char[] punctuation = " .,!?-\\/|~!@#$%^&*()[]_+=;:".ToCharArray();
TextBoxSpellChecker tbSpellChecker = this.radSpellChecker1.GetControlSpellChecker(typeof(RadTextBoxControl)) as TextBoxSpellChecker;

FieldInfo fi = typeof(TextBoxSpellChecker).GetField("punctuation",
    System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
fi.SetValue(tbSpellChecker, punctuation);