You're right - I wasn't considering how narrow his case is and was perhaps overgeneralizing, particularly about the cause.

Seems we agree the better solution for column_index_+1 doesn't exist is to call it out instead of stealthily append a new column, but the why the newer models have that behavior is indeed speculative.

It a bit echos the conundrum from back in the PC days where IBM hardware was the de-facto standard, and companies building "compatible" hardware had to decide whether to be compatible with the spec, or compatible with every detail of the implementation, including buggy behavior, of which OFC some software took advantage. So, do they build to be "compatible" or "bug-compatible"?

Was the ChatGPT v4 response highlighting the missing column a bug or failure to shoot straight? Not sure I'd characterize it that way, but there definitely could be many other reasons for the change in behavior (other than training on lower-skilled programmers' inputs) — we really have to consider that as a conjecture on the author's part.