Since updating to Mountain Lion, or perhaps later versions of Lion, I’ve seen a bug where the Mac freezes when coming out of the screen saver. There’s no way to unlock the screen. It has not actually kernel panicked, as applications continue to run and I can ssh in, however the Mac is essentially unusable until it’s been shut down—via ssh, as that’s a bit cleaner than using the power button. This problem didn’t occur every time I used the screen saver, but at its peak it was happening nearly once a day.
I tried different screen savers, but that didn’t help. What did help was disabling the screen saver entirely (setting it to “Start after: Never”) and instead using Energy Saver’s “Display sleep” setting to lock the screen. (I had been using this before, but with Energy Saver set to kick in after a longer delay than the screen saver.) Using these settings for nearly a month, the problem has not recurred.
However, I’m still seeing various display waking problems that have been happening since Lion. Sometimes both screens remain blank (but with the backlight on) until after I’ve entered my password and pressed Return. Other times, the internal display wakes but the external one doesn’t. In that situation, the workaround is to unplug and replug the display. The window positions get all messed up, then Moom fixes them automatically.
All of this is to say that I still think that 10.6.8 was the pinnacle of Mac OS X reliability.
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