In "Enter 77", Sayid, Kate, John visit "The Arrow" station. While in the station Sayid points a Mosin Nagant 91/30 rifle at one of the others (Beatrice Klugh). During the scene, the bolt of the rifle alternates between the locked (ready to fire) and open/unlocked position. Not only is this a continuity error, but if fired in this unlocked position, the results would be a catastrophic failure of the rifle.
See screen-shot http://i38.tinypic.com/4zyy9u.jpg for proof.