UrlRewriter.net exception - anyone ever seen this?

Question

This showed up in our error log over the weekend. Has anyone ever seen this?

System.ArgumentException: Item has already been added. Key in dictionary: 'RewritingXtoY'  Key being added: 'RewritingXtoY'
   at System.Collections.Hashtable.Insert(Object key, Object nvalue, Boolean add)
   at Intelligencia.UrlRewriter.Utilities.MessageProvider.FormatString(Message message, Object[] args)
   at Intelligencia.UrlRewriter.RewriterEngine.Rewrite()
   at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)