src/Pure/General/exn.scala

         case other: User_Error => message == other.getMessage
         case _ => false
       }
     override def hashCode: Int = message.hashCode
 
-    override def toString: String = "ERROR(" + message + ")"
+    override def toString: String = "ERROR(" + Output.clean_yxml(message) + ")"
   }
 
   object ERROR
   {
     def apply(message: String): User_Error = new User_Error(message)