When there are words highlighted as wrong and triggering the Finish Spellchecking without correcting any words, subsequent editing of words inside the sentence causes random cursor replacement. 

Workaround:

<telerik:RadEditor ID="RadEditor1" runat="server" OnClientLoad="OnClientLoad">
    <Content>Whether you need a mere Textbox with Google-like spellchecker, or a Word-like content authoring environment, the result is the same: clean <strong>XHTML</strong></Content>
</telerik:RadEditor>

<script>
    function OnClientLoad(sender, args) {
        sender.add_spellCheckLoaded(spellCheckLoaded)
    }

    function spellCheckLoaded(sender) {
        var editor = sender;
        var spell = editor.get_ajaxSpellCheck();

        spell.add_spellCheckEnd(function (sender, args) {
            editor.set_html(editor.get_html(true));
        });
    }
</script>