This error is often encountered when SharePoint OM is used to update site/web/list objects from within a web context.  Some thing so basic as the code below could fail:

using (SPSite site = new SPSite("http://moss"))
{
    using (SPWeb web = site.OpenWeb())
    {
        SPList list = web.Lists["List1"];
        list.Title = "List2";
        list.Update();
        web.Update();
    }
}

Setting the "AllowUnsafeUpdates" properties of both the SPSite and SPWeb objects to "true", mostly will resolve this issue out if you are using a code similar to above within a webpart or an ASP.NET web application.  When you are running from the context of a separate web application make sure the application pool identity running your ASP.NET web application is the same as the one that's running SharePoint's site.  Below is a code that fixes the "security validation for this page is invalid" error.

using (SPSite site = new SPSite("http://moss"))
{
    site.AllowUnsafeUpdates = true;
    using (SPWeb web = site.OpenWeb())
    {
        web.AllowUnsafeUpdates = true;
        SPList list = web.Lists["List1"];
        list.Title = "List2";
        list.Update();
        web.Update();
    }
}

Thought we might actually prevent some support calls by posting this :)