FYI, I added a new site, mapped it to a new domain, and Live Preview failed.
Changed back to the subdomain, and Live Preview works.
Nothing in the Apache error logs.
Here is the access log of a FAILED LIVE PREVIEW:
108.73.53.2 - - [12/Jul/2012:20:59:26 +0000] "GET /wp-admin/customize.php?theme=firstyme HTTP/1.1" 200 16773 "http://foo.mypencil.net/wp-admin/themes.php" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_4) AppleWebKit/534.57.2 (KHTML, like Gecko) Version/5.1.7 Safari/534.57.2"
108.73.53.2 - - [12/Jul/2012:20:59:27 +0000] "GET /wp-content/themes/firstyme/img/header-default.jpg HTTP/1.1" 200 19895 "http://foo.mypencil.net/wp-admin/customize.php?theme=firstyme" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_4) AppleWebKit/534.57.2 (KHTML, like Gecko) Version/5.1.7 Safari/534.57.2"
108.73.53.2 - - [12/Jul/2012:20:59:27 +0000] "POST / HTTP/1.1" 301 23 "http://foo.mypencil.net/wp-admin/customize.php?theme=firstyme" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_4) AppleWebKit/534.57.2 (KHTML, like Gecko) Version/5.1.7 Safari/534.57.2"
Here is the access log of a SUCCESSFUL live preview:
108.73.53.2 - - [12/Jul/2012:20:58:54 +0000] "GET /wp-admin/customize.php?theme=blaskan HTTP/1.1" 200 17258 "http://foo.mypencil.net/wp-admin/customize.php?theme=catch-box" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_4) AppleWebKit/534.57.2 (KHTML, like Gecko) Version/5.1.7 Safari/534.57.2"
108.73.53.2 - - [12/Jul/2012:20:58:55 +0000] "POST / HTTP/1.1" 200 11807 "http://foo.mypencil.net/wp-admin/customize.php?theme=blaskan" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_4) AppleWebKit/534.57.2 (KHTML, like Gecko) Version/5.1.7 Safari/534.57.2"
108.73.53.2 - - [12/Jul/2012:20:58:56 +0000] "GET /wp-content/themes/blaskan/js/libs/jquery.fitvids.js?ver=3.4.1 HTTP/1.1" 200 2543 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_4) AppleWebKit/534.57.2 (KHTML, like Gecko) Version/5.1.7 Safari/534.57.2"
108.73.53.2 - - [12/Jul/2012:20:58:56 +0000] "GET /wp-content/themes/blaskan/js/libs/modernizr.min.js?ver=3.4.1 HTTP/1.1" 200 14990 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_4) AppleWebKit/534.57.2 (KHTML, like Gecko) Version/5.1.7 Safari/534.57.2"
108.73.53.2 - - [12/Jul/2012:20:58:56 +0000] "GET /wp-content/themes/blaskan/js/mylibs/helper.js?ver=3.4.1 HTTP/1.1" 200 4657 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_4) AppleWebKit/534.57.2 (KHTML, like Gecko) Version/5.1.7 Safari/534.57.2"
108.73.53.2 - - [12/Jul/2012:20:58:56 +0000] "GET /wp-content/themes/blaskan/js/script.js?ver=3.4.1 HTTP/1.1" 200 1180 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_4) AppleWebKit/534.57.2 (KHTML, like Gecko) Version/5.1.7 Safari/534.57.2"
108.73.53.2 - - [12/Jul/2012:20:58:56 +0000] "GET /wp-content/themes/blaskan/framework.css?ver=3.4.1 HTTP/1.1" 200 6658 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_4) AppleWebKit/534.57.2 (KHTML, like Gecko) Version/5.1.7 Safari/534.57.2"
108.73.53.2 - - [12/Jul/2012:20:58:56 +0000] "GET /wp-content/themes/blaskan/style.css?ver=3.4.1 HTTP/1.1" 200 37383 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_4) AppleWebKit/534.57.2 (KHTML, like Gecko) Version/5.1.7 Safari/534.57.2"
108.73.53.2 - - [12/Jul/2012:20:58:56 +0000] "GET /wp-content/themes/blaskan/fonts/league_gothic-webfont.woff HTTP/1.1" 200 24300 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_4) AppleWebKit/534.57.2 (KHTML, like Gecko) Version/5.1.7 Safari/534.57.2"