A global retailer you've definitely heard of that used to own stores in Germany spent a lot of time preparing for Y2K. This was a long and painful process, but it got done in time.
But problems still slipped through. These problems ended up not being that big and visible because a large percentage of the code base had just been recently vetted, and a useful minority of that had been recently updated. Every single piece had clear and current ownership.
Lastly, there was an enormous amount of vigilance on The Big Night. Most of the Information Systems Division was on eight hour shifts, with big overlap between shifts at key hours (midnight in the US), and everyone else was on call.
As midnight rolled through Germany, all of the point of sale systems stopped working.
This was immediately noticed (the stores weren't open, but managers were there, exercising their systems), the responsible team started looking into it within minutes, a fix was identified, implemented and tested within tens of minutes, and rolled out in less than an hour.
Pre-Y2K effort and planning was vital; during-Y2K focus, along with the indirect fruits of the prior work, was also critical.