Of 17 hives, many replaced last spring due to winter losses, I had two occurrences of agressive
hives.  In both cases, people nearby were being stung, not only those who approached the hive
itself.
  In one case there were four colonies in the bee yard, on a farm. Field workers (not close to the
hive) were being stung.  One of the four hives exhibited a strong "flash back" of bees when I
inserted a gloved hand, and I moved that hive.  The problem vanished.  I also moved the other hive.
  Requeening was tough becuause this was mid-summer and both hives were so full of bees that I after
moving them I had to separate the brood boxes, moving them 50 feet from the supers, and leaving
empty brood boxes under the supers.  The field bees returned to the colony with the now empty brood
boxes, while the original brood boxes (now with many fewer bees) gave me a chance to find the queen
and requeen. A week or so later I re-combined the brood box under the original hive.
  Both hives exhibited normal behavior into the fall.