# power outages while in the middle of a firmware flash update (this problem has been alleviated a lot by programs which refuse to update without a proper, clean, isolated power supply)
# improperly tested firmware from vendors which led to other complications
# changes to firmware which was made impossible due to some weird intermediate problem
# firmware loads which were impossible due to some strange electronic fault which had to be rectified first (for telecommunications equipment that I've come across clean/sanitary conditions seem to be core to their optimal function)
# flash updates that were only possible through the use of an off board programming device (think about Flash EEPROM programmers)
# firmware updates that were only possible via a minimum version of Operating System (completely different to the old days when things were done via DOS and a bootable disk though safeguards were obviously minimal)
- if you can't figure out how to hit DFU mode try the traditional 'technology Jedi master' trick as follows... My book on 'Cloud and Internet Security' via Google or Amazon online stores provides further details on bypassing (and defending) chip based security for those who are interested...