That's the official verdict on the March 2014 slide in Oso, WA. that killed 43 people and seriously injured ten others, wiping out the Steelhead Haven neighborhood formerly located across the river and half a mile inland from the site. Heavy rainfall in the area prior to the event appears to have mobilized the site of a 2006 slide first, then moments later, the site of an adjacent ancient landslide containing glacial deposits. Such slides appear to occur in the area once every 400+ years.
Studies in previous decades indicated a high landslide risk for the Oso area, the researchers found, but they noted that it does not appear there was any publicly communicated understanding that debris from a landslide could run as far across the valley as it did in March. In addition to the fatalities, that event seriously injured at least 10 people and caused damage estimated at more than $50 million.
It's also been recently discovered that the city of Seattle is at high risk for landslides as well, even in areas previously thought not to be prone to them. That's because there's a fault that runs right through the city, where even a minor crustal quake will likely trigger catastrophic landslides.
In retrospect, this should hardly seem surprising; the present footprint of the city was riddled with canyons when development began in the mid 1800s, and in fact there's an entire steam locomotive buried in one of them still. Most of the city is built upon ridges and fill, which may have seemed like a good idea at the time but may well prove to have been a huge error.