In response, Microsoft has posted a discussion thread at the Windows Insider site that helps fill us in on what went wrong, what's being changed in the future, and whether or not current bricked phones can be fixed.
At the forefront, Microsoft believes that what caused certain devices - namely the Lumia 520 and 521 - to brick is that the data blocks being sent to the device were too large. In response, Microsoft has adjusted its recovery tool (WPRT) to send 128KB chunks rather than 2MB. Further, it'll hard-cap the data transfer speed to 5MB/s, instead of the usual 8MB/s.
Those owning a bricked device can attempt to recover it using the WPRT tool, making sure to restart it so that it forces an update. It sounds like you'll have better luck in recovery if your phone is showing a red Nokia logo, as in all such instances, the phone is in the same state. What's a little murkier is if your phone shows a black screen and simply won't boot up. Microsoft says phones in that state could actually be stuck at different spots, so while some might be able to recover their phone, others may not.
Whether or not you have success with recovery, Microsoft encourages you to relay your experiences so that it can nail down a fix sooner than later. If you are the unfortunate owner of a newly-bricked Lumia, hit up the link below and chime in with your experience.