"Give 'em an inch, and they'll take a mile."
But on further thought I refrained, and even found something lacking in the proverb in general.
When you give someone an inch, generally you are inclined to do so because you believe they have at least some good qualities. One important such quality is a conscience.
And when somebody with a conscience has been offered an inch, and find that they have somehow ended up taking a mile, they will tend to feel bad about it. So they will have to take action to avoid goin' down the road feelin' bad.
Occasionally they will give back the mile and apologise for their fault. But now they are almost as poorly off as before, and are still feelin' at least kind of crummy after the apology. This may explain the relative rarity of this event.
The other approach is to convince themselves that they were actually owed more than a mile anyway, and showed noble generosity in only taking one. It is all the inch-giver's fault! Which, in turn, may explain gratitude's notorious propensity for turning rancid faster than fresh milk left out in the sun.
This leads directly to my proposed new and improved proverbial wisdom:
"Give 'em an inch, and they'll take a mile - and resent you for not giving a league."
No ingrates were harmed in the production of this post. Fat lot of thanks I'll get for that!