event log warning - on checkout/confirm page

I'm seeing some recurring items in my event log for this warning (I've XXXX'ed out sensitve parts). It seems to only show up when a logged-in user is on the confirm checkout page.

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Event code: 3005
Event message: An unhandled exception has occurred.
Event time: 10/3/2011 1:24:21 PM
Event time (UTC): 10/3/2011 8:24:21 PM
Event ID: 35efdb4d7c5e4963842368b96032012f
Event sequence: 2506
Event occurrence: 1
Event detail code: 0

Application information:
    Application domain: /LM/W3SVC/3/ROOT-1-129621342038072500
    Trust level: Full
    Application Virtual Path: /
    Application Path: C:\inetpub\XXXXX\
    Machine name: xxxxxxx

Process information:
    Process ID: 9840
    Process name: w3wp.exe
    Account name: IIS APPPOOL\xxxxxx

Exception information:
    Exception type: HttpException
    Exception message: Server cannot set status after HTTP headers have been sent.
   at System.Web.HttpResponse.set_StatusCode(Int32 value)
   at System.Web.Mvc.HandleErrorAttribute.OnException(ExceptionContext filterContext)
   at System.Web.Mvc.ControllerActionInvoker.InvokeExceptionFilters(ControllerContext controllerContext, IList`1 filters, Exception exception)
   at System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName)
   at System.Web.Mvc.Controller.ExecuteCore()
   at System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext)
   at System.Web.Mvc.MvcHandler.<>c__DisplayClass6.<>c__DisplayClassb.<BeginProcessRequest>b__5()
   at System.Web.Mvc.Async.AsyncResultWrapper.<>c__DisplayClass1.<MakeVoidDelegate>b__0()
   at System.Web.Mvc.MvcHandler.<>c__DisplayClasse.<EndProcessRequest>b__d()
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)



Request information:
    Request URL: http://xxxxxx.xxx/checkout/confirm
    Request path: /checkout/confirm
    User host address: xx.xx.xx.xx
    User: [email protected]
    Is authenticated: True
    Authentication Type: Forms
    Thread account name: IIS APPPOOL\xxxxxx

Thread information:
    Thread ID: 73
    Thread account name: IIS APPPOOL\xxxxxx
    Is impersonating: False
    Stack trace:    at System.Web.HttpResponse.set_StatusCode(Int32 value)
   at System.Web.Mvc.HandleErrorAttribute.OnException(ExceptionContext filterContext)
   at System.Web.Mvc.ControllerActionInvoker.InvokeExceptionFilters(ControllerContext controllerContext, IList`1 filters, Exception exception)
   at System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName)
   at System.Web.Mvc.Controller.ExecuteCore()
   at System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext)
   at System.Web.Mvc.MvcHandler.<>c__DisplayClass6.<>c__DisplayClassb.<BeginProcessRequest>b__5()
   at System.Web.Mvc.Async.AsyncResultWrapper.<>c__DisplayClass1.<MakeVoidDelegate>b__0()
   at System.Web.Mvc.MvcHandler.<>c__DisplayClasse.<EndProcessRequest>b__d()
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)


Custom event details: