According to a press release, the phenomena below could occur in the aforementioned instances:
1. The applicable EF-X500 flash unit does not properly respond to turn on from stand-by mode.
2. The alkaline batteries may become increasingly hot. If you try to take the batteries out of the EF-X500 flash unit without turning the flash off and/or disconnecting the flash unit from the camera and letting the batteries cool down, a skin burn could occur.
A firmware upgrade is now available to address and prevent this potential occurrence in relevant EF-X500 units.
Accordingly, Fujifilm recommend that applicable product owners upgrade their EF-X500 flash firmware to version 1.02 which is posted on the firmware download site.
The serial numbers of the flashes which requires this firmware upgrade can be checked at http://www.fujifilm.com/news/n170308.html. If the serial number is not applicable, the firmware of the EF-X500 flash unit does not need upgrading.