I am pretty sure it was! Spain in general out of BOS via JFK with JFK-CDG in F.

BTW I recalled a case which may have similar causes as this one. SU fares BKK-NYC in PE which booked to J on BKK-MOW part. That was a fare differential constuction but for some reason the differential was zero (for the upgrade from cheapest premium economy A to Z, I, and D classes; and some relatively small amounts to upgrade to C and J). The glitch was GDS specific.
After that SU simply prohibited to use its fares for differential construction:D If you dont understand it - BAN it:D

Seems many airlines do not understans how differential pricing works or at least do not pay attention to the setting which shall keep fares clean from such glitches.

A hint: seems my original guess was right and the issue is relatively cheap 'BU - business unrestricted' fare (wrong type probably). This works with NYC-CAI ZFF fare - without a differential on certain week days, not a good price (5.9K) but a good confirmation to my theory,

Does anybody see any 77W? I could have found only 8am departure CDG-JFK which does not sound appealing to me.

no, it is not bookable. i don't know why this is not removed.

not sure about CAI, but I know the fare I posted is bookable (with F seat selection, etc), just nobody knows if DL/AF will cancel your ticket or downgrade to business. the difference with this fare is that it's DL coded, not AF, so it could have different results than the stories posted upthread

if you look at the DL ZN1H56D1 fare rules, it clearly states the following:
https://cimg1.ibsrv.net/gimg/www.fly...b498f22fd8.png

https://cimg3.ibsrv.net/gimg/www.fly...5bb5fe928c.png

Here is detail on permitted booking codes. No F on overwater segments permitted, just within USA/Canada. There is a booking class mapping error with some of the booking engines.

https://cimg2.ibsrv.net/gimg/www.fly...c3d99d2edf.png

Fare application is just for information, this text is never linked to any coding ( NOTE - THE FOLLOWING TEXT IS INFORMATIONAL AND NOT VALIDATED FOR AUTOPRICING.). It never matters what this part of fare rules is saying.

But the linked seatmap does matter of course:)

Again to me its a fare differential lost for an upgrade from Z to F for unknown reason rather than a coding error. But I saw coding errors too (when a fare rule text validated for autopricing does not correspond to the actual coding), so it could be that too.

it's a common and frequently seen error that happens with DL codeshares on AF flights. This isn't the even close to first time this has been an issue with these fares.