In Sherlock S04E01, when Lestrade asks John about his fatherhood and if he is getting any sleep, John makes a reference joke (Sherlock being the baby) and Lestrade joins in too. But Sherlock doesn't seem to understand their conversation. Following is the transcript:
LESTRADE: You’re at the beck and call of a screaming, demanding baby, woken up at all hours to obey his every whim. (He looks pointedly at Sherlock.) Must feel very different.
SHERLOCK: I’m sorry, what?
JOHN: Yes, well, you know how it is. All you do is clean up their mess, pat them on the head.
SHERLOCK: Are you two having a little joke?
JOHN: Never a word of thanks. Can’t even tell people’s faces apart.
SHERLOCK: This is a joke, isn’t it?
LESTRADE: Then it’s all, ‘Ooh, aren’t you clever? You’re so, so clever!’
SHERLOCK: Is it about me?
LESTRADE (as an aside to John): I think he needs winding.
JOHN: You know, I think that really might be it.
SHERLOCK: No, don’t get it.
But in S03E02, Sherlock himself admits to John and Mary that they won't need him around, as they are expecting a real child. Following is the transcript:
SHERLOCK: You’re already the best parents in the world. Look at all the practice you’ve had!
JOHN: What practice?
SHERLOCK: Well, you’re hardly gonna need me around now that you’ve got a real baby on the way.
So, why didn't Sherlock(of all the people in the world), understand John's joke about him (Sherlock) being a little child?