This morning, Adobe kindly alerted me of an update available for Flash to take it to version 188.8.131.52.
The update breaks Flash.
DO NOT run this update, at least until Adobe solves whatever is causing the plugin to fail.
Several readers of Overthought.org have reported that this update completely breaks Flash for Safari on OS X 10.9 Mavericks. I have also personally experienced this issue.
If you run into a problem with your newly updated Flash install, read the rest of the post. Hopefully, it will help. [Via @stumark and @jordanmerrick]