Alfred Hitchcock’s comedy-thriller The Trouble with Harry (1955), amidst all its talk of murder and romance, has a fun little exchange of sociolinguistic interest between John Forsythe (‘Sam Marlowe’) and Edmund Gwenn (‘Capt. Albert Wiles’):
Marlowe’s correction is notable for being relatively polite. Those who correct others’ speech uninvited often do so in a rude and judgemental way. Marlowe corrects Wiles gently and off-handedly, as though automatically correcting a child. Indeed, Wiles doesn’t even notice and reacts as if Marlowe had merely echoed him. For good measure he adds another nonstandard usage: past tense say for said.
That Miles doesn’t pick up on the prescriptive nudge also chimes with what happens when children have their speech corrected – they tend to repeat what they said rather than immediately adopt the ‘proper’ form. Abby Kaplan, in her excellent book about language myths, Women Talk More than Men, reviews the research and concludes:
Some parents tend to repeat or expand on their children’s utterances, but it is unclear whether children actually use this kind of feedback to correct their own speech. Since there are societies in which this kind of interaction is rare, it is unlikely that repetitions and expansions are absolutely necessary for language acquisition.
Of course, Captain Wiles has already fully acquired his language: it’s just that the variety or dialect he uses differs in some respects from standardized English, prompting Marlowe’s useless intervention.
The script for The Trouble with Harry was written by John Michael Hayes. I don’t know if the same exchange appears in the source novel by Jack Trevor Story, but Hitchcock obviously liked it. He featured another linguistic allusion, to Alfred Korzybski and his General Semantics, in The Words – I mean, The Birds:
Hitchcock’s interest in usage also manifests in a letter he wrote to Ernest Lehman, writer of North by Northwest, in which he wondered, in a parenthetical aside, if his use of while should be whilst. I covered the whilst, amongst, amidst issue in a previous post.
Sentence first 
“For good measure he adds another nonstandard usage: past tense say for said.” My instinct is to interpret the captain’s error as present tense “say” for present progressive “am saying”: I’m not totally sure why I think that is the error, rather than “say” for “said”, except that it seems more natural for the captain to be emphasising the continuous aspect of his question – he continues to ask “who done it?”, rather than asking it once. Whatever, it’s still an error!
Jack Trevor Story – what a wildly under-rated writer he is today.
That is a possible interpretation, and one I considered before deciding not to equivocate for once in describing it. It’s too late for the screenwriter to shed any light on it, unfortunately. Another curious aspect that I found, in rewatching the clip a bunch of times, is that it sounds rather like “That’s what I’ve say”. So who knows.
“Who done it?” is not incorrect. At least not for an American. In US English it’s perfectly correct, an idiomatic Hollywood usage. It is an idiom for a crime genre, a “who done it.” The Englishman Hitchcock is playing with this idiom, making it, ironically, into a form of regional dialect. But every American would get the irony of correcting the idiom with stiff proper grammar. The irony is on many levels, including, to Americans, confirming the stereotype of an English person as much too proper. And the irony of Hitchcock amusing himself by identifying Americans as speakers of a regional dialect.
Of course, that’s another layer to it.