I think I've posted this before, but there have been two occasions that I booked with an alternative airline due to BA's IT not working.

First, I needed to be in London on fixed dates coming up quite soon. BA's website and app didn't work, so in frustration and needing those dates I booked KLM. What a discovery - getting to somewhere near LCY took about the same with KLM via AMS than flying to LHR with BA.

I still fly BA more than KLM short-haul but that's because I'm focussed on using up all my Avios before any eventual devaluation.

Second, we used to fly to Asia via HKG on BA with a stop-over. This could never be done on BA's website even with a functioning multi-city page. First, it would only show a few of the daily flights from HKG onwards, and second as soon as the transit turned into a stop-over, it would throw an error. I would phone BA and book. One time, after waiting over half an our on the phone, I gave up, paid a bit more, and flew Cathay J. What an experience. I'll never fly BA long-haul to Asia again unless they're noticeably cheaper than the competition.

So, due to BA's crappy IT, they've lost our annual long-haul J flights and around 1/3 of my short-haul.