I don't know if this is Facebook's case, but when I've seen stuff like that happening, it was because a bunch of (otherwise smart) developers were far too arrogant about the quality of their own work, too derogatory and, to some degree, too superficial about the quality of other programmers' work, and -- perhaps fatally -- too driven by the can-do-no-matter-what attitude that is so obnoxiously prevalent in today's corporate world, at the expense of common sense. Between the technical debt and the management pressure, I've seen a bunch of people refusing to admit it's their fault and cleverly save their asses (and earn a hefty bonus on one or two occasions) by doing something any sane programmer would have, in fact, fired them for.