The 270th prohibition is that we are forbidden from leaving someone who is trapped under his burden and stuck in the road. Rather, we must help him unload the burden until he can adjust it, then help him lift it, either onto his own back1 or onto his animal, as explained in the details of this mitzvah.
The source of this prohibition is G‑d's statement,2 "[If you see the donkey of someone you hate lying under its load,] you might want to refrain from helping him, [but you must make every effort to help him unload it]."
In the words of the Mechilta: "From the verse 'you might want to refrain from helping him, but you must make every effort to help him [unload it].' we learn that the person transgresses both a positive commandment and a prohibition."
There is another prohibition that speaks independently regarding this idea, as said in the verse in Deuteronomy,3 "You may not see your brother's donkey [or ox fallen...]." The Sifri says, "The verse 'You may not see your brother's donkey...' constitutes a prohibition," and later says that "The verse4 'When you see your brother's donkey...' constitutes a positive commandment."
The details of this mitzvah are also explained in the second chapter of tractate Bava Metzia.5
