Am travelling to Connecticut (BDL) in February and ran into issues doing so. Despite the NZ website offering 3 return segments, the booking crapped out at payment 4 times. I finally rang the reservation line and they explained that on non-NZ segments, the booking engine doesn't check availability on other carriers. Turns out that all the connections ex BDL that day are hooped by the sole UA segment to ORD that connects to NZ's ORD/IAH/SFO/LAX flights.

In the 21st century I expect an airline's online booking engine to check for availability when offering segments. Or, at the very least, having a substantive/precise message WHY a booking is crapping out.

Countdown to the Air NZ apologists in 3....2.....1