It is a common scenario users to start selecting text and accidental trigger mouseup event outside of the content area. Still, the focus is inside RadEditor and InlineEditCompleted event should not fire.

Possible workaround:

<telerik:RadEditor runat="server" ID="RadEditor1" EditType="Inline"
    OnClientLoad="OnClientLoad" OnClientInlineEditCompleted="OnClientInlineEditCompleted">
    <Content>
       some text
    </Content>
</telerik:RadEditor>

<script>
    function OnClientInlineEditCompleted(sender, args) {
        if (sender._editCompleted) {
            alert("Fired!");
        }
    }

    function OnClientLoad(sender, args) {
        $telerik.$(document).on("mouseup", function (e) {
            if (e.target === sender.get_contentArea() || e.target === sender.get_textArea()) {
                sender._editCompleted = true;
            }
        })

        $telerik.$(document).on("mousedown", function (e) {
            if (e.target === sender.get_contentArea() || e.target === sender.get_textArea()) {
                sender._editCompleted = false;
            } else {
                sender._editCompleted = true;
            }
        })
    }
</script>