The Sheldon version
Sheldon: “I have a question. If someone tells me, ‘You have £20. Get whatever food you want,’ what exactly am I permitted to purchase?”
Friend: “Whatever you want, as long as it’s within £20.”
Sheldon: “Excellent. Then I have established the parameters of the agreement. £20 is the upper financial limit, and the choice of food is at my discretion.”
Friend: “Yes.”
Sheldon: “So if I purchase two jacket potatoes, each with a different topping, and the total cost is £19.50, I have complied with the agreement.”
Friend: “Well... I thought you'd only get one.”
Sheldon: “That is irrelevant.”
Friend: “Why?”
Sheldon: “Because you did not communicate that restriction. You communicated a budget, not a quantity limitation. If your actual intention was ‘You may have one jacket potato, provided it costs no more than £20,’ then that is what you should have said.”
Friend: “But surely you knew what I meant?”
Sheldon: “No. I knew what you said. Those are demonstrably different things.”
Friend: “I just assumed you'd get one.”
Sheldon: “Then we have identified the problem. You substituted an assumption for a condition.”
Friend: “It wasn't meant to be a big deal.”
Sheldon: “Then why make the sarcastic comment two days later?”
Friend: “Because I thought two was excessive.”
Sheldon: “Then you should have established a restriction before granting permission. Otherwise you are effectively changing the rules after the experiment has concluded.”
Friend: “You’re being ridiculous.”
Sheldon: “No. I am being extraordinarily literal because you gave me literal instructions.”
The problem isn't really the second jacket potato. The problem is that the person apparently asked for the relevant boundary — “What is the budget?” — and was given an answer that appeared to establish the boundary: £20 and choose what you want.
They then acted within that stated boundary. Discovering afterwards that the giver had privately intended an additional restriction creates an entirely avoidable problem.
There is an important distinction between permission and permission subject to undisclosed conditions. If I say, “You can choose whatever you like for dinner, up to £20,” the ordinary interpretation is that I have delegated the choice to you, subject to the £20 ceiling.
If I actually mean, “You can choose whatever you like, but I expect you to buy only one main item,” then that restriction is part of the permission and should be communicated.
Otherwise, asking permission becomes almost meaningless.
The person seeking permission cannot make an informed decision because the supposedly relevant rules are concealed in the other person's head.
The fairest response from the friend would therefore have been something like:
“Sorry, I should have been clearer. I was thinking of one jacket potato rather than two. If I'd realised you wanted two, I'd have said so beforehand.”
That acknowledges the actual communication failure.