The only time this has ever happened to me (and it was using two PowerMacs) was when I was attempting to boot in Target Disk Mode (is that what you mean when you say "t mode"?).
The Mac with the higher version of OS has to be the one that goes in to Disk Mode and the other has to play host.
I've never had anything like that message before, but it's pretty cool (though admittedly not from your perspective).
Sorry I can't be of more help. (8+(
Mark