A couple of people have told me that it's a bad for plants to be watered in the middle of a sunny day, i.e. it's not just that you waste more water due to evaporation, but that there are actually bad effects on the plants itself. I've heard this claim specifically for watering grass.
The explanation I heard is that it causes the plants to open up pores or otherwise behave as if it were raining, but the fact that the sun is actually shining leads to burning or other damage. But there are certainly other possible explanations.
Can it cause harm to plants to water them under a hot sun?