That is why Andreas M. Antonopoulos, a founding partner at Nemertes Research, a technology research company in Mokena, Ill., says, “The threat is complexity itself.”

Image GLITCH Relatives and friends waited for travelers at the Los Angeles airport in August after U.S. Customs computers failed and passengers were kept on planes. Credit... Dan Steinberg/Associated Press

Change is the fuel of business, but it also introduces complexity, Mr. Antonopoulos said, whether by bringing together incompatible computer networks or simply by growing beyond the network’s ability to keep up.

“We have gone from fairly simple computing architectures to massively distributed, massively interconnected and interdependent networks,” he said, adding that as a result, flaws have become increasingly hard to predict or spot. Simpler systems could be understood and their behavior characterized, he said, but greater complexity brings unintended consequences.

“On the scale we do it, it’s more like forecasting weather,” he said.

Kenneth M. Ritchhart, the chief information officer for the customs and border agency, agreed that complexity was at the heart of the problem at the Los Angeles airport. “As we move from stovepipes to interdependent systems,” he said, “it becomes increasingly difficult to identify and correct problems.”

At first, the agency thought the source of the trouble was routers, not the network cards. “Many times the problems you see that you try to correct are not the root causes of the problem,” he said.

And even though his department takes the threat of hacking and malicious cyberintruders seriously, he said, “I’ve got a list of 16 things that I try to address in terms of outages — only one of them is cyber- or malicious attacks.” Others include national power failures, data corruption and physical attacks on facilities.

In the case of Skype, the company — which says it has more than 220 million users, with millions online at any time — was deluged on Aug. 16 with login attempts by computers that had restarted after downloading a security update for Microsoft’s Windows operating system. A company employee, Villu Arak, posted a note online that blamed a “massive restart of our users’ computers across the globe within a very short time frame” for the 48-hour failure, saying it had overtaxed the network. Though the company has software to “self-heal” in such situations, “this event revealed a previously unseen software bug” in the program that allocates computing resources.