Most of what is exchanged in conversation between interlocutors is referential (i. e., strictly oriented toward content), but occasionally––and depending on a person’s variable disposition toward the norms of speech––one interlocutor may insert a correction of or comment on the grammatical side of what the other interlocutor has uttered. Parents routinely correct the speech errors of children; some parents are more scrupulous than others, insisting in some cases on adherence to norms that may be traditional or conservative rather than current.
To the extent that this kind of interpolation may interrupt the flow of speech, some speakers, while silently noting the irruption of an error, may choose to refrain from overtly correcting it, while others may habitually do so regardless of its force. Some linguistic purists take a special delight in correcting their interlocutors, as is the case with Y-H-B’s brother Jacobus Primus, who recently jumped at the chance when hearing the question “who called who?” (the colloquial norm in American English) from the lips of an otherwise strict adherent of linguistic normativity. He then went on to recount the case of a former coworker who blithely ignored being corrected for the erroneous locution “between you and I” and went on to repeat the mistake habitually.
Changes in language often start out as violations of the norm but when adopted by a significant proportion of the speech community cease to be regarded as errors and assume the status of grammatically unexceptional specimens. The “who called who?” example is an illustration of just such a trajectory.