help with unhandled exception

 Over the weekend our application threw an unhandled exception twice.  This morning I tested on Dev, Test, and Production and I got no errors. Can anyone suggest what may have caused this or how I might find the cause?
 
 
Last Error Stack Trace:


System.Web.HttpUnhandledException: Exception of type
'System.Web.HttpUnhandledException' was thrown. --->
System.NullReferenceException: Object reference not set to an instance of an
object.


   at
CheckOutACollege.ReadySetGo.Ready.InterestSurvey.btnSave_Click(Object sender,
EventArgs e)


   at
System.Web.UI.WebControls.Button.OnClick(EventArgs e)


   at
System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument)


& ...