If a <hr> tag is inside a <strong> and the strong tag has no text in it, then the editor will crash and just say "Loading editor". I've tested this bug with 4.51.5, 4.61.5 and CMS and they all have the bug. Im guessing the previous versions of episerver also has the bug.

 Two javascripts error will be displayed:

TableEdit.prototype.SwitchToDesignMode = function(skipContentRefresh)
{
 var content;

 GetEditorIFrame(this.editorID).style.display = '';

 if (this.editorState.Mode == EDITORMODE_HTML)
 {
  this.editorHtmlArea.style.display = 'none'; 
  content = this.editorHtmlArea.value;
 }
 else
 {
  content = this.editorDocument.body.innerHTML;
 }

 if (content.length == 0)
 {
  if (this.ParagraphIsDiv)
   content = '<DIV></DIV>';
  else
   content = '<P></P>';
  this.editorDocument.body.innerHTML = content;
 }

 if (!skipContentRefresh)
 {
  var repairForDesignMode = true;
  if (this.NeedsRepairing(content, repairForDesignMode))
  {
   content = this.RepairContent(content, repairForDesignMode);
  }

  if (this.htmlAreaContentBeforeChanges)
  {
   if (this.htmlAreaContentBeforeChanges != content)
    this.editorDocument.body.innerHTML = content;

   this.htmlAreaContentBeforeChanges = null;
  }
  else
   this.editorDocument.body.innerHTML = content;
 }

 this.AddTableDesignElements();
 this.ResetEditor();
 this.editorState.Mode = EDITORMODE_DESIGN;
 this.RestoreCursor();
}

TableEdit.prototype.SwitchToDesignMode = function(skipContentRefresh)
{
 var content;

 GetEditorIFrame(this.editorID).style.display = '';

 if (this.editorState.Mode == EDITORMODE_HTML)
 {
  this.editorHtmlArea.style.display = 'none'; 
  content = this.editorHtmlArea.value;
 }
 else
 {
  content = this.editorDocument.body.innerHTML;
 }

 if (content.length == 0)
 {
  if (this.ParagraphIsDiv)
   content = '<DIV></DIV>';
  else
   content = '<P></P>';
  this.editorDocument.body.innerHTML = content;
 }

 if (!skipContentRefresh)
 {
  var repairForDesignMode = true;
  if (this.NeedsRepairing(content, repairForDesignMode))
  {
   content = this.RepairContent(content, repairForDesignMode); // <--- here is the error
  }

  if (this.htmlAreaContentBeforeChanges)
  {
   if (this.htmlAreaContentBeforeChanges != content)
    this.editorDocument.body.innerHTML = content;

   this.htmlAreaContentBeforeChanges = null;
  }
  else
   this.editorDocument.body.innerHTML = content;
 }

 this.AddTableDesignElements();
 this.ResetEditor();
 this.editorState.Mode = EDITORMODE_DESIGN;
 this.RestoreCursor();
}

AND