Q My late 2012 Mac mini with 2.6 GHz i7 processor had been working fine, but for no apparent reason started to flash the screen instead of sounding an alert. I have tried to fix this in the Accessibility and Sound panes to no avail, although regular sound output still works fine. What is wrong?
A The most likely cause is that the preference .plist containing that setting has become mangled, and changes made are not being properly saved to it. The fix is to trash the respective .plist from one of your Preferences folders, inside a Library folder.
In the first instance, try trashing ~/Library/Preferences/com.apple.universalaccess.plist, restarting, and setting the Accessibility pane up again.
Updated from the original, which was first published in MacUser volume 29 issue 4, 2013.