Earlier this week, much of the internet ground to a halt when the servers that power them suddenly vanished. The servers were part of S3, Amazon’s popular web hosting service, and when they went down they took several big services with them. Quora, Trello, and IFTTT were among the sites affected by the disruption. The servers came back online more than four hours later, but not before totally ruining the UK celebration of AWSome Day.

Now we know how it happened. In a note posted to customers today, Amazon revealed the cause of the problem: a typo.

On Tuesday morning, members of the S3 team were debugging the billing system. As part of that, the team needed to take a small number of servers offline. “Unfortunately, one of the inputs to the command was entered incorrectly and a larger set of servers was removed than intended,” Amazon said. “The servers that were inadvertently removed supported two other S3 subsystems.”

The subsystems were important. One of them “manages the metadata and location information of all S3 objects in the region,” Amazon said. Without it, services that depend on it couldn’t perform basic data retrieval and storage tasks.

“We want to apologize for the impact this event caused for our customers.”

After accidentally taking the servers offline, the various systems had to do “a full restart,” which apparently takes longer than it does on your laptop. While S3 was down, a variety of other Amazon web services stopped functioning, including Amazon’s Elastic Compute Cloud (EC2), which is also popular with internet companies that need to rapidly expand their storage.

Amazon said S3 was designed to be able to handle losing a few servers. What it had more trouble handling was the massive restart. “S3 has experienced massive growth over the last several years and the process of restarting these services and running the necessary safety checks to validate the integrity of the metadata took longer than expected,” the company said.

As a result, Amazon said it is making changes to S3 to enable its systems to recover more quickly. It’s also declaring war on typos. In the future, the company said, engineers will no longer be able to remove capacity from S3 if it would take subsystems below a certain threshold of server capacity.

It’s also making a change to the AWS Service Health Dashboard. During the outage, the dashboard embarrassingly showed all services running green, because the dashboard itself was dependent on S3. The next time S3 goes down, the dashboard should function properly, the company said.

“We want to apologize for the impact this event caused for our customers,” the company said. “We will do everything we can to learn from this event and use it to improve our availability even further.”