Since Monday's WWDC keynote, Apple has been pushing out a range of minor updates which ready OS X for new hardware. Among them is a fairly trivial 537 KB Thunderbolt update—which happens to be causing some massive boot issues.
The update, Thunderbolt Software Update 1.2, provides support for Apple's new Thunderbolt to Gigabit Ethernet Adapter, which is designed for use with the MacBook Air and the new Retina MacBook Pro.
However, TidBITS has noticed that it's causing major problems for some users, whose computers are now unable to boot:
"Although it's unclear if all Thunderbolt-equipped Macs are affected, a number of people have reported that installing the update causes a variety of strange boot-related problems (kernel panics, getting stuck at the gray boot screen, "Unexpected error" messages, and more). Regardless of the details, in each case, it seems that the Mac is rendered unusable, even thunderstruck. I've now heard from some people for whom the update did not cause any problems, but it's still unclear what the difference may be between those who are and are not suffering boot failures."