Here is the thing, depending on how you give the drug only a certain amount gets into the blood stream. Oral has the least amount of absorption, then intramuscular, then IV. I want to say the percentages are about 40% (Oral), 60% (IM), and 100% (IV), they are not exact because I don't feel like pulling down my pharmacology notes.
Now lets say you need to give him 10mg of a medication IM, that means only 6mg is going to be used by his body. You give him the injection and get vein on accident and don't know. You have just given him almost twice the normal dose and depending on the medication that could very well be lethal!