This patch omits the 'printable version' link on the printable version. It fixes the problem in a default install, but includes/Skin.php and skins/disabled/MonoBookCBT.php may also need attention.
I noticed Googlebot indexing "/wiki/index.php?title=Foo&printable=yes&printable=yes"
Printable page URLs are constructed by adding &printable=yes to the current link.
The printable-version link on a printable page just adds another &printable=yes,
and that page adds another, and so on, indefinitely. The printable-version link
on a printable-version page isn't visible in the browser, but it's in the document.