From Fujifilm - a problem was found with the latest firmware update version 4.2 for the Fuji X-T1 mirrorless camera:
Notice for customers using FUJIFILM X-T1
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.X-T1 Ver.4.21Download Page: X-T1 / X-T1 Graphite Silver Edition Camera Body Firmware Update Ver.4.21