I've been advising about unticketed reservations pretty much since I joined FT and this forum. My perception - since there is no data - is that incidence levels have gone down, hopefully in part due to all the advice in this forum about checking e-ticket numbers. Having said that, in that downward trend there are still people getting caught out by it, and had the OP been on say an AA or IB service connecting into BA then they may well only find out at check-in. The "Why can't I OLCI / Check-in" thread has a lot of advice and background about this and it remains important to check ticketing if OLCI fails. But the one thing that is definitely the case is that BA have been using Agile to reboot their crumbling IT infrastructure, and when it comes to Minimum Viable Product, the stress is on Minimum. So if there has been IT issues during booking - more than possible during this Nexus IT project - then the risk of being unticketed goes up. Hopefully that is temporary and as I say I've spent less time advising about this over recent years.

Now that background isn't very interesting if you are stranded somewhere, though people should be reassured that it always gets fixed somehow or other, even if the experience is not pleasant. It's not an experience I've ever had on BA, since I always check ticketing, but I have had on other airlines, this is by no means a BA unique issue. Eurowings used to be notorious about this if you had originally booked via Lufthansa, to take one example of very many. The fact that the OP got a message here is interesting, albeit a clumsy message, since that would not have happened a few years ago.