Stargate SG-1: Season 3, Episode 9

3 - 9: Rules of Engagement

Upon exiting the Stargate, SG-1 finds itself in the midst of battle. A group of SG soldiers battle a Jaffa army. Believing the soldiers to be the missing-in-action SG-11 team, O'Neill and the others provide assistance. Much to their surprise, the mystery SG team turns their weapons on SG-1.