In reunion there's a part of the episode where ronon walks in on mckay and sam and then asks if carter was in cahrge. mckay then introduces ronon to sam as if they had never met and sam does the same.
However they HAVE met before at the end of lifelines. While they were landing atlantis in the ocean sam was in the control room along with mckay, teyla AND ronon. Additionally carter would have been on atlantis for a day or so while they were traveling in hyperspace and possibly for about another day after they landed (or there abouts) So in all taht time no one never boterhed to introduce sam and ronon and they seemingly forgot about eachother completely.
I don't know whether anyone has brought this up before but i had just realised that so i though i'd share.