I've been debugging some checkout issues. I noticed that several of them were fixed in Magento 2.0.7, so I went in to test our payment process using that version.
As I tested my integration with Autorize.NET DPM gateway, my checkout page froze in the "loading" stage. The order was successfully received and Authorize.NET debug log showed the information. However, something was happening after the response came back.
Sometimes I would get a window saying "Cannot place order". Sometimes it would just get stuck. However, at no point I would get more detailed information, as a developer, to what's going on.
I traced down the issue to the vendor/magento/module-authorizenet/Controller/Directpost/Payment/Place.php file (I'm using a Composer based install). The following method would be executed: