Recently, my Fuji X-T1 began freezing up on me randomly. Turning the camera off did not even correct the issue. I had to pop the battery out to reset the camera -- and, of course, after using it a bit, the same thing would happen. I was ready to send the camera back to Fuji but began by searching the internet to see if anyone else had the same problem. Sure enough they did.
Apparently, the last 4.20 firmware update in December 17, 2015 was causing the problem. Fuji has issued a firmware replacement, version 4.21 to correct the problem. This is a stop-gap measure to roll back the camera to version 4.10 until Fuji can come up with a replacement fix.
Here is Fuji's statement on this firmware problem:
"We have found a malfunction in the firmware ver.4.20 launched on December 17 2015, which rarely causes freeze when shooting in AF-C mode. So we will suspend the disclosure of this firmware right away.
We apologize for your inconvenience which this may cause.
We will post an upgraded firmware around the end of January 2016 to solve it.
As a tentative solution for customers who have upgraded their cameras to firmware ver.4.20, we have prepared the new firmware version of 4.21 whose functions are same as the previous version of 4.10. Please update your cameras to ver.4.21 to avoid the malfunction."
If you are having this freezing problem, you can download the 4.21 temporary patch firmware here.