I was running 3.10 and rebooted with the 3.12 package in the root directory.
I found that as it would reboot it would get to the point where it shows the login prompt, keyboard working at this point, and then give the double beep. At this point the keyboard stops working entirely and will completely halt. Nothing changes on the screen - I continue to see the login prompt, cursor blinking, but be unable to interact via the keyboard and it does not respond to any arp/cdp/etc.
FFR I installed 2.9.49 from a disk I had (which recovered the config from 3.12) and then upgraded to 3.11, which continued to recover the config!
Hardware:
d815eea motherboard
p3 667mhz
intel NICs
3com 905b NICs
senao 600mw - EPI-3601S w/ Atheros AR5213 @ http://forum.mikrotik.com/posting.php?mode=post&f=2