Continuity mistake: In the scene where Michael and Sawyer are eating some fish on the beach (with Jin working on the raft) a pregnant Claire has a quick conversation with them and heads into the forest. About 20 seconds later Kate runs into Michael and Sawyer, and runs into the forest at full speed. Kate later finds Claire in labour, who somehow had gotten further out than Kate, even though Kate was running at high speed in the woods. There is no way a pregnant girl could have beat Kate to that position, even considering how long it took Jin to find them from the beach.
Revealing mistake: Jack's left arm is bandaged from Boone's blood transfusion. After the shot of Shannon, as she cries over Boone, Jack sits on the beach in the wideshot and the bandage is on his right arm, but in the following shots it's on his left. Yet another flipped shot.