I think that attributing to bees any strict obedience to any "rule" is far
off the mark.
Bees clearly are very good at improvisation.

While it has been claimed that bees can only remember one combination of
flower appearance, site, and odor for any one 15-min or so timeslot, it is
also well-known that bees react to "footprint pheromone" by moving to
another bloom.  In an area of mixed blooms, it is certainly possible that
more than one bloom will be producing nectar at the same time of day, or
that there might be overlapping periods of maximum nectar production.

So, one bloom gets exploited, the foragers find slim pickings, and are
forced to gather nectar and pollen from another set of flowers in  the same
area.  The foragers certainty would not return empty-handed (empty-legged?)
to the hive if there was nectar and pollen to be had nearby. 

A conservatory garden with beds of mixed plants would be a good place to
test this, but I'd have to use my day-glo chalk-dust kit to mark bees with
the color of the bloom they first forage upon, and keep a video camera
running to capture all the visits.  

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