One of our customers had a problem with their F50 server hanging with a 518 on the LED display when it was rebooted.
Dropping into maintenance mode and running fsck on the filesystems did not come back with any error messages and the server also did not respond to a reboot. Thanks to a 5 year old posting on Google at http://tinyurl.com/jexo we also formatted the log filesystem, rebooted the box and we were able to get the machine up and running again. This was a big relief as last nights backup had failed (due to/causing the system hang) and they had no AIX cd’s to rebuild from.