Authorize.net has implemented a fix, and the issue has cleared.
Posted Jan 21, 2016 - 09:41 EST
Monitoring
We are monitoring the issue.
Posted Jan 21, 2016 - 09:00 EST
Identified
The authorize.net gateway is experiencing issues. All shops using this gateway will experience checkout issues. You can follow the status of the incident at https://status.authorize.net/