So I've tried to simplify this to 3 main areas of immediate cannula failure. I believe that If you know how to fail, you will know exactly what to do not to fail. When supervising doctors just starting to learn cannulation, I understand that failure occurs. However, if failure occurs and the student knows why they failed, this allows them to improve next time with deliberate attention to detail on their next attempt! 1) miss the vein altogether (no flashback of blood is observed) 2) puncture the deep wall or side wall 3) The needle and cannula isn't advanced far enough prior to advancing the plastic cannula. that's it!