If you are thinking of designing your own failsafe, have a look at one of my webpages all about the subject:

http://homepages.which.net/~paul.hills/Circuits/Failsafe/Failsafe.htmlhttp://homepages.which.net/~paul.hil.../Failsafe.html

It describes how to make circuits that do fail safe. This includes what happens when transistors and other components fail. Failsafes that cannot cope with any single component failure in their circuit contradict their names!