It happened some time ago. At the time, one of the now defunct Judges, Surgeon, was attempting to reveal Sabra la Tau's past to us. To that end, he gave a few people from every pair of teams a memory from that past. Each team used to be a nation, back when Sabra la Tau's people lived on its surface. The past memories we were given each held some information on the nature of those past countries. It was a one-time event, but in that case those who were given the past memories still knew which memory they were supposed to take.