Pe'ah Chapter 5 Mishnah 3

One may not turn on the water wheel (prior to the poor taking leket), according to R' Meir. But the Sages permit it since he can (be careful in preventing a loss to the poor).

Berachos, Chapter 1, Mishnah 1

From when do we read the "Shema" in the evening? From the time that the Cohanim enter to eat of their terumah, until the end of the first watch--so says Rabi Eliezer. And the Sages say: until midnight. Rabban Gamliel says: until dawn.

The story is told that his sons came from a party and said to him, "We have not recited the 'Shema.'" He said to them, "If it's not yet dawn, you are required to recite it."

And not only this example, but everytime the Sages say, "Until midnight," it's requirement lasts until dawn. Bringing the fats and limbs--it's mitzvah lasts until dawn. Everything eaten for one day--it's mitzvah lasts until dawn. If so, why did the Sages say, "Until midnight"? In order to prevent people from transgressing.