I was reading this rather interesting article on Windows's horrible old Win32 API, versus Apple's tactic of effectively scrapping MacOS pre-10 in favour of an all-new API and operating system.
When, suddenly, Safari's UI elements started to vanish. I've never seen this before, but I suspect that it's something along the lines of the higher-profile Windows-running-out-of-window-descriptors problem that tends to crop up if you leave XP running for a few weeks.
I blame Eclipse; it's been running for a few days and does tend to cause the Windows problem on Windows.
All was well after a reboot, of course, but it was irritating and disconcerting nonetheless. Bad, naughty Apple!