OVM Firmware Boot
I've recently been tasked to re-purpose a T5120 (i.e. Fresh Sol 10 load, apps, etc.), and apparently, this box was intended for use as a SPARC-based Oracle VM Server in a past life. So, in it's current state, it boots into a firmware-based linux kernel immediately upon connecting power. I should note here that no action involving the power switch is required, merely connecting AC power starts this.
So far, I've not been able to find anyway to interrupt or disable this process; and, of course, no one remembers the required login credentials for this once this linux boot is complete.