Q: What very essential piece of caution, seems missing most of the time, in femoral arterial line cannulation?

Answer: The femoral artery should always be accessed approximately 1 inch below the inguinal ligament, where it can be easily compressed. Unfortunately, many femoral arterial lines are placed/entered very high - near the inguinal ligament. In case of supra-inguinal bleed/injury, it becomes hard to control the bleeding by compression.