I had similar cracks on the rear rim of my 2006 Roubais. These were 10 year old Mavic Open Sport rims. Someone said incorrect spoke tension, but since the wheels always stayed true, I never checked for that. There were two cracks on adjacent spokes, so I don't think they were drive side specific. I noticed it while cleaning the bike after a gear cable change. No idea how long they were broken, since the bike rode fine. I got a deal on a set of Reynolds Stratus Elites, so the cracked rim and it's partner are hanging on the garage wall. The Mavics are double wall construction, and might have been safe to ride but why chance it?