JDK 7's sort function is stricter about verifying that implementations of the comparison function actually obey their contract. It will even helpfully throw an exception when this is not the case:

java.lang.IllegalArgumentException: Comparison method violates its general contract!

The root cause is this gem:

(defn shuffle
      "Shuffle coll"
      [coll]
      (sort-by (fn [_] (long)) coll))

Which does not work on JDK7 since TimSort is clever enough to detect that the ordering function is behaving inconsistently, resulting in the previously mentioned exception.

Also, this is just a bad idea:

A variant of the above method that has seen some use in languages that support sorting with user-specified comparison functions is to shuffle a list by sorting it with a comparison function that returns random values. However, this is an extremely bad method: it is very likely to produce highly non-uniform distributions, which in addition depends heavily on the sorting algorithm used.

(wikipedia)