UpdatePanel doesn't like the ViewState being tampered with!

Symptom:
Previous instances of RadAjaxPanel were replaced with UpdatePanel.  Suddenly, controls within that page no longer postback.

Sample Code:
<asp:UpdatePanel ID="UpdatePanel3" runat="server">
    <ContentTemplate>
        <asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" />
        <asp:Button ID="Button2" runat="server" Text="Button" OnClick="Button2_Click" />
    </ContentTemplate>
</asp:UpdatePanel>

protected void Button1_Click(object sender, EventArgs e)
{
Button1.Text = DateTime.Now.ToString();
}

protected void Button2_Click(object sender, EventArgs e)
{
Button2.Text = DateTime.Now.ToString();
}

When alternating clicks between Button1 and Button2, I would expect that the clicked button would update with the current time.  Instead, the button values were clearing.


Solution:
It turns out this was specific to a certain page.  On a normal page, it worked fine.  On the page with these symptoms, I discovered it was related to a solution that implimented due to a large ViewState.  I was overriding SavePageStateToPersistenceMedium and LoadPageStateFromPersistenceMedium and that was causing the UpdatePanel to fail when saving the ViewState information to the Cache.