timebomb-comment, from Runa (now Staples Innovation Lab), makes sure that comments stay up-to-date. This is pretty serious business!

(defmacro timebomb-comment
  "Used to comment things out that we want to force ourselves to come
   back to by a certain point in time in order to resolve later."
  [timestamp-long & body]
  `(if (<= ~timestamp-long (System/currentTimeMillis))
     (throw (Exception. "Timebomb comment has passed its due date."))
     (comment ~@body)))

I’ve definitely felt pain from old & busted comments, and this kind of thing would sure motivate me to keep anything remotely old out of the codebase.