Final answer:
The last tube of blood collected should be removed before needle removal to prevent air embolism, which is a serious and potentially fatal condition.
Step-by-step explanation:
The last tube of blood collected should be removed from the needle/adapter unit before needle removal to prevent air embolism. An air embolism can occur if air is allowed to enter the blood system, which can be life-threatening. It is not related to accidental injection of blood into the patient, causing the patient to faint, or blood contamination in this context.