_PendingCallbacks[...].async is null or not an object BUG

_PendingCallbacks[...].async is null or not an object error can be related to a bug in the javascript that VS creates for you to deal with callbacks. You can alleviate PendingCallback errors by checking all of your own javascript for any variables named i. Replace them and you should be golden.

Show and hide a panel with javascript

* Make note of the Z-INDEX on the row. This will allow the visible panel to appear above other items on the page.
In the HTML
<tr style="Z-INDEX: 10; POSITION: absolute">
  <td>
    <asp:Panel Runat="server" ID="ThePanel" BorderWidth="1px" BorderStyle=Solid>
    <asp:Literal Runat="server" ID="TheResults"></asp:Literal>
</asp:Panel>
  </td>
</tr>

In the js file
function ShowPanel(target)
{
  if (target != null)
  {
    target.style.display = '';
  }
}

function HidePanel(target)
{
  if (target != null)
  {
    target.style.display = 'none';
  }
}

In the code behind
protected override void OnPreRender(EventArgs e)
{
  this.TheLabel.Attributes.Add("onmouseover", "ShowPanel("+this.ThePanel.ID+");");
  this.TheLabel.Attributes.Add("onmouseout", "HidePanel("+this.ThePanel.ID+");");
  this.ThePanel.Style.Add("display", "none");

  base.OnPreRender (e);
}