Like many commuters, Dan Yalon relies on Waze to get from his home in Tel Aviv to his job (he’s an Executive VP at 3D printing leader Stratasys) in Rehovot with the least hassle possible. Enter a destination in the dialog box of the Israel-made crowdsourced traffic and driving app, and Waze will present you with the route of least resistance. Many drivers swear by Waze, which has saved over 20 million drivers in dozens of countries around the world millions of commuting hours, the company claims and many users who give testimonials on the app’s websites confirm.

Waze bases its motoring magic on two main pillars: the app’s use of your smartphone’s GPS chip to determine where you are, and reports from users on traffic problems, road hazards, accidents, —  even where the cops are hiding out to find speeders. Waze notes your current location on the road and checks your progress. If it’s near the speed limit, it will declare the road clear, and recommend it to others.

But if you’re moving slowly, Waze knows there’s a tie-up. By extrapolating how long it will take you to get through the traffic and checking out the traffic situation on other roads along your route, Waze will estimate how long it will take till you get to your destination. Many drivers rely on Waze for that information, and build their commute around it, as does Yalon.

With heavy rains causing all sorts of traffic problems countrywide on Tuesday, Yalon expected the worst — but was pleasantly surprised to learn from Waze that the commute would take him no longer than the usual 30 minutes. He wondered about that; the radio was full of reports of delays, accidents, and closed roads. But Waze would never steer him wrong – would it?

It turns out that it would, and it did. Yalon, like hundreds of thousands of other commuters, uses the Ayalon Expressway to navigate the traffic-choked length of Tel Aviv. Running from north to south, the Ayalon is a vital artery that for many offers the only viable option for transiting Tel Aviv during rush hour.

Unfortunately for Yalon, the Ayalon was closed Tuesday due to the overflow of the Ayalon River, a usually dry concrete-bottom waterway that has sprung to life in the past few days thanks to torrents of rain. It happens once in awhile, and the news was prominently featured on radio news reports. But somehow, the message didn’t make it to Waze, which blithely recommended to Yalon and many other drivers to use the Ayalon as usual and presented it as completely clear of traffic.

Which it was, but not in a good way. As a result, Yalon, who usually finds Waze more trustworthy and up-to-the-minute than the radio’s traffic reports, found himself stuck in a monumental traffic jam with near-gridlock on all the roads leading to and from the highway. It took him three hours to extract himself from that, at which point he turned tail and went home, a victim of one of the worst storms to hit Israel in years — and a major blooper on the part of Waze.

How could this happen? Numerous calls to Waze’s office in Ra’anana went unanswered Tuesday morning (perhaps the staff couldn’t get to work either?). But Grig Davidovitz, a new media expert and Times of Israel consultant, offered a possible explanation. “In the Waze system, if there are no signs of heavy traffic — meaning drivers moving more slowly than expected — then the app considers the situation to be okay.” But with no one driving on the Ayalon on Tuesday, there were no reports of jams so the app thought it should send drivers there.

“It’s a good example of what can happen when an algorithm does its job unsupervised,” Davidovitz added.

Irate drivers using the Waze chat option Tuesday morning called for a weather feature, or at least a “closed road” feature. But for now, Yalon may have to start paying attention to those radio traffic reports.