No one had tested this until Adam Egri and associates from Budapest came along. Their research is described in the December issue of Natural History magazine.
They first put drops of water on ginkgo and maple leaves that were exposed to full sunlight. The water drops spread out over the leaf surface without focusing the sun and the evaporation actually ended up cooling the leaves. There was no damage done.
Next they tried it on the floating fern. As you know from seeing rain drops on a freshly waxed car, the surface tension of water can cause nicely rounded drops on a waxed surface like many leaves possess, This plant has wax hairs which can keep the water drops domed and above the leaf surface. By keeping the leaf horizontal and out of the wind, they were able to demonstrate some scorching on the leaf.
What does this mean in the real world? They pointed out that this condition rarely occurs in the real world. With leaves hanging at all angles and wind moving them, water runs off rapidly. Ii is unlikely that a significant amount of damage would occur on even the most waxy leaves.
Another common belief is that watering at night increases plant mildew, although I am not aware of any studies. Watering in the morning to reduce evaporation and prevent sunburn to the gardener is still probably not a bad idea.
Research is constantly breaking down ideas that "everybody knows". Next thing you know, someone will prove that handling toads doesn't cause warts!