It also delivered to patch 25 security bugs in Snow Leopard.

Before Wednesday, Apple's last Snow Leopard security update shipped on May 9.

Because of yesterday's update, only a small minority -- about 13% -- of all Macs are running a retired edition of OS X. If Apple had done the expected and not patched Snow Leopard, that number would have , according to the latest statistics on operating system usage from metrics company Net Applications.

"Apple realized they need to support Snow Leopard because there are still so many active installations," said Wolfgang Kandek, CTO of Qualys, in reply to questions posed via instant message. "It might be a Windows XP-like effect. Snow Leopard works really well, and many customers have not migrated off it."

As Kandek noted, Microsoft has had difficulty convincing users of the 11-year-old Windows XP to upgrade to a new edition; by Net Applications' numbers, XP powered 46% of all Windows PCs last month.