The term "failbreak" mainly refers to an incomplete or otherwise flawed jailbreak that cannot run Mobile Substrate properly. Some failbreaks cannot be released to the public for various reasons, so "failbreak" is also sometimes used to refer to any jailbreak that cannot be released to the public, whether or not that jailbreak is complete. Sometimes people also call fake jailbreaks "failbreaks". Since this word has multiple meanings, it's confusing to try to use it in conversation, so it's best to avoid it.
saurik has said that "the term was actually first used years ago by chpwn on a released jailbreak as there was something wrong with it that caused Substrate to only work in some processes; I was then later using it with regards to jailbreaks where the kernel patches didn't support the various memory protection changes required by C Substrate. I provide a tool called 'vmcheck' that people developing jailbreaks use to 'unit test' their patches, and when it fails... well, that's a 'failbreak'."
chpwn showed a screenshot of his new iPhone 5 running Cydia shortly after its launch on September 19, 2012. Shortly afterward, usage of this term picked up, and on October 19, 2012, planetbeing tweeted that he upgraded the "failbreak" with a kernel exploit so that tweaks actually work on the iPhone 5, to make it "almost a full tethered jailbreak.". chpwn explained a failbreak on Twitter.
- iOS 4.2.1 (Jailbreak Monte)
- Access to iOS 4.2b3 beta for device
- iOS 6.x on A5, A5 Rev A A5X, A6 and A6X