All the other answers here are wrong. It was the Boeing 737-Max.

They fit bigger, more fuel efficient engines on it that changed the flight characteristics, compared to previous 737s. And so rather than have pilots recertify on this as a new model (lots of flight hours, can’t switch back), they designed software to basically make the aircraft seem to behave like the old model.

And so a bug in the cheaper version of the software, combined with a faulty sensor, would cause the software to take over and try to override the pilots and dive downward instead of pulling up. Two crashes happened within 5 months, to aircraft that were pretty much brand new.

It was grounded for a while as Boeing fixed the software and hardware issues, and, more importantly, updated all the training and reference materials for pilots so that they were aware of this basically secret setting that could kill everyone.