I had an interesting experience with this once.
Got J tickets for myself and DW from FlyingBlue JFK-SVO on DL SVO-TLV on SU. When I booked the FlyingBlue tickets, I asked to purchase an infant ticket, got a whole runaround, and then was told to call DL in order to purchase that ticket. When the email arrived with the J tickets, there was also a ticket for the infant included for which I paid a grant total of $0.
DL agent at JFK had a hard time checking us in, but eventually did some kind of override, and gave us a boarding pass for the infant. At SVO it took even longer, but eventually we got a boarding pass for the infant there too (some kind of manually issued one).
Coming back, I had to hack a few tickets in order to be able to purchase the infant ticket at a better price (he might still have an unused JFK-TLV segment on that ticket - never checked).