There seems to be two very different ways that the phrase "social contract" is used.
The first is an unspoken conduct agreement between two people. If you hire a great guitar player to teach you guitar lessons once a week, are you allowed to cancel? Is your instructor? How much notice? Is it okay if he's drunk or halfway-preoccupied during your lesson?
How much formality is there? If you don't do the recommended lesson from last time, how disappointed will your instructor be?
How prepared must you both be?
This is a social contract that's actually a social contract. Sure, there's edge cases - even if it's expected that both of you are always at the lesson on-time, prepared, and ready to go immediately, there's still an exemption if you have a family emergency or serious illness or whatever.
Link: http://ebooks.adelaide.edu.au/j/james/william/talks/chapter8.html (found it via http://everydaysystems.com/)
All our life, so far as it has definite form, is but a mass of habits,— practical, emotional, and intellectual,— systematically organized for our weal or woe, and bearing us irresistibly toward our destiny, whatever the latter may be.
Habit is thus a second nature, or rather, as the Duke of Wellington said, it is ‘ten times nature,’