On most days, the Internet and the myriad systems it powers can be counted on to work well enough. But security experts say problems are inevitable, whether because of hacking, human error, broken cables, buggy code or other unforeseen issues.

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NEW YORK — It was a rough day for tech: The nation’s second-biggest airline, its oldest stock exchange and its most prominent business newspaper all suffered technology problems that upended service for parts of the day.

Government officials said it did not appear that the incidents were related, or the result of sabotage, counter to an endless stream of jokes and conspiracy theories posted on Facebook and Twitter — and even the suspicions of FBI Director James Comey.

“In my business, you don’t love coincidences,” Comey told Congress on Wednesday. “But it does appear that there is not a cyber-intrusion involved.”

First a “router issue” at United Airlines suspended all of the company’s flights for nearly two hours, starting at 8 a.m. EDT, leading to 800 flight delays and 60 cancellations. Then at 11:32 a.m. EDT a “technical problem” at the New York Stock Exchange halted trading. As those two failures happened, The Wall Street Journal’s website, WSJ.com, had “technical difficulties” that sent readers to a temporary site while the paper worked to fix the problem.

“The problem is humans can’t keep up with all the technology they have created,” said Avivah Litan, an analyst at Gartner. “It’s becoming unmanageable by the human brain. Our best hope may be that computers eventually will become smart enough to maintain themselves.”

Government and company officials said that although the causes of the outages were technical and likely unrelated, the shutdowns nonetheless raise concerns about the vulnerability of vital organizations that can be easily crippled by malfunctions or cyberattacks.

As the world becomes more connected, such events expose serious risks for countries, companies and individuals who depend so heavily on fragile technology — often a mash-up of neglected old-fashioned processes and cutting-edge systems. Electricity grids, credit cards, social media, email, public transportation and GPS all have become indispensable to everyday modern life.

“These are incredibly complicated systems. There are lots and lots of failure modes that are not thoroughly understood,” said Jeff Schmidt, chief executive and founder of JAS Global Advisors, a security consulting firm. “Because the systems act so quickly, you have this really increased potential for cascading failures.”

Technology makes life easier and the economy more efficient, allowing for nearly instantaneous flow of information and communication and for remote control of far-flung operations. Until it fails. And technology problems like those that occurred Wednesday and that temporarily knock out vital services and conveniences of modern life are likely to become more common as computers and other electronic devices increasingly connect together over the Internet.

For United, it was the second major technical failure in two months. On June 2, the airline had to halt all takeoffs in the United States because of what it described as computer-automation issues.

When ordering a ground stop Wednesday, the airline blamed “network connectivity.”

United spokeswoman Jennifer Dohm said a network router problem was the culprit. Security experts said the cause of such problems can take time to investigate.

United has had three chief information officers (CIOs) since 2011, with the current CIO joining last September. The airline also struggled through a series of computer outages in 2012 after switching to the passenger-information system of Continental Airlines after that carrier merged in 2010 with United.

After the merger, United elected to combine many computer systems and frequent-flier programs all at once. Executives believed that any disruptions would thus be short-lived. By contrast, Delta and Northwest integrated their systems in stages after a 2008 merger, and American Airlines is taking Delta’s same go-slow approach as it absorbs US Airways.

Indeed, it may be that we are rushing to push technology into business operations and our daily lives before it is fully ready, some experts said. Lillian Ablon, a technology researcher for the RAND Corp., says the confluence of breakdowns should be interpreted as a wake-up call to companies and engineers to program their networks to protect them against inevitable glitches and malicious attacks by outsiders.

“Instead of just letting the technology rush ahead of us and then trying to catch up in terms of privacy and security, we should be baking those things into the systems from the start,” she said. “We need to be a little smarter on how we are coding things.”

As for The Wall Street Journal, the newspaper’s home page went down about the same time as the stock-exchange outage but was back up within an hour. A spokeswoman for The Wall Street Journal said the outage was still being investigated late Wednesday.

The length of Wednesday’s two other outages was disconcerting, Gartner’s Litan said.

It took the New York Stock Exchange until 3:10 p.m. — just over 3½ hours — to resume trading. “I think everyone needs to assume technology is going to go down sometimes, but you should be resilient enough to quickly recover from the outage within a half-hour, if not a few minutes,” Litan said.

Still, being deprived of technology for a few hours is a reminder that it is still clearly better than the alternative.

“When you’ve got 6 or 7 billion shares a day (trading), I’m not sure you’ve got any other choice,” said Larry Tabb, founder and chief executive officer of the Tabb Group, a financial-research firm that focuses on market structure and trading. “I can’t imaging going back to paper tickets and floor trading. If Google goes down are you going to have a bunch of people with encyclopedias looking up answers for people? The cat is out of the bag.”

Correction: An earlier version of this story incorrectly described United Airlines as the biggest U.S. airline. It’s the second biggest.