I had one RHEL 4 server (out of a half dozen identically configured systems) crash at exactly the moment the leap second should have been inserted. The logs run up to 18:59:59 CST (UTC-06:00) and the system froze, when all my other Linux systems logged they were inserting a leap second. I have read reports of some Debian systems having a similar problem. The leap second code is probably one of the least tested areas of the Linux kernel (there have only been 8 leap seconds since Linux was started); there is probably some race condition related to stepping the clock that only some systems hit.
I have also read reports of problems with Oracle RAC (not stand-alone Oracle) crashing at the leap second.