MySQL 5.5 (Percona) assertion failure log.. what would cause this?

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http://serverfault.com – 256GB RAM, 64 Core , AMD running Ubuntu 12.04 with Percona MySQL 5.5.28. Below is the assertion failure. We just had a second assertion failure (different "in file", position, etc) while running a large set of inserts. After the first failure, MySQL restarted after a reboot only - after continuously looping on the same error after trying to recover. I decided to do a mysqlcheck with -o for optimize. Since these are all Innodb tables (very large tables, 60+GB) this would do an alter table on all tables. In the middle of this , the below assertion failure happened again: 121115 22:30:31 (HowTos)