Programmer on several games/engines from 1989 - 1998, primarily on Amiga, Sega Saturn, Sony Playstation 1.
Even back then, the occasional bug could prove very difficult to track down -- particularly if the bug resided in a low-level subsystem that interacted only probabilistically with the rest of the engine, such as an Interrupt Service Routine which in some circumstance was trashing a register that wasn't properly saved, leading to what could be very intermittent and seemingly random odd behavior or crashes of the game.
Today's engines tend to be coded at a slightly higher level, but are considerably larger and more complex systems of software than the ones we developed.
So it is no surprise at all that there will sometimes be bugs that will take an extended length of time for the DayZ devs to reproduce, investigate, characterize, identify, and fix -- no matter how badly they might have wished to release to stable.
And as Dean recently pointed out, the longer they defer the release to stable, the more painful for their team, who will be dealing with increasingly complex code branching and merging as development of new features continues in parallel with the experimental branch which to some degree becomes stuck in limbo while the remaining game-breaking bugs are solved, before it can finally be pushed to stable.
tl;dr: The DayZ team themselves want to get 0.42 onto stable at least as badly as any of the complainers in these threads.