Just 5 minutes ago i finished a Left 4 Dead campaign in single player with the intent of getting the Safety First achievement. I played through the whole thing with no friendly fire and we never went back to a checkpoint (not sure if that matters or not.) Upon completing the campaign i watched the statistics screen and saw that neither me or any of the AI had any type of friendly fire incidents. I did not, however, receive the achievement. So my question is: Does anyone know why this happened and has this ever happened to any of you?