Tom Dotan::

The blue screen of death has been a dreaded symbol of technological failure since Microsoft’s Windows became the world’s dominant operating system in the 1990s.

On Friday, it showed up on millions of computers around the world at once, highlighting both Microsoft’s continued ubiquity in workplaces and decades-old design choices that allowed the actions of a little-known software company to disable millions of Windows machines. Some security professionals also say Microsoft MSFT -0.74%decrease; red down pointing triangle hasn’t taken the vulnerability of its software seriously enough.

Microsoft said in a blog post Saturday that 8.5 million Windows machines were hit, or less than 1% of its global footprint. That number was enough to bring down the operations of major businesses across industries including healthcare, media and restaurants.

The effects continued to reverberate in airports Saturday, as U.S. carriers canceled close to 2,000 flights, compared with 3,400 Friday. Delta, which accounted for more than half the canceled flights Saturday, has been trying to make sure it has crews to cover flights and told pilots at hub airports to depart when planes are fully boarded and ready to safely go, no matter the scheduled departure time.

Friday’s outage was caused by a buggy update sent to corporate clients by CrowdStrike CRWD -11.10%decrease; red down pointing triangle, one of hundreds of cybersecurity firms that have built a business promising to make Windows more secure. Microsoft has its own competing product, called Windows Defender.