We had an AS/400 at my previous job.

I worked there for 6 years, and saw it go down twice unexpectedly. Well, one of those times was a result of operator error (resetting the SECOFR password to something he promptly forgot, requiring a manual IPL). The other time was the result of using an obscure serial port debugging tool.

OS/400 is rock solid... I'm not really sure what you're on about.