A host that we refer a number of clients to recently decided it was a good idea to upgrade to PHP 5.3.x on one of our client's VPS's without informing us.
Suddenly, clicking through to a content page was WSOD, but only for certain content types. Frantic debugging begins, and of course there's no project hours to support this level of disaster.
Admin menu broke. Empty white space where it should be, and no "admin-menu" div in the source html.
Emfield broke. Empty white space where videos should play.
Too much other stuff broke to mention.
And then a simple phpinfo() surprise: PHP 5.3 magically appears a few days ago.
Another lesson in "never ignore the obvious" when debugging your suddenly broken site.