Based on MySQL 5.5.27, including all the bug fixes in it, Percona Server 5.5.27-28.1 is now the current stable release in the 5.5 series. All of Percona‘s software is open-source and free, all the details of the release can be found in the 5.5.27-28.1 milestone at Launchpad.
Percona Server 5.5.27-28.0 would crash or deadlock in XtraDB buffer pool code. This was caused by incorrect mutex handling in porting of the recently introduced InnoDB code to XtraDB. Bug fixed #1038225 (Laurynas Biveinis).