People get hurt all the time without noticing it. Occasionally, people get totally destroyed without noticing it, either.
5A Taser Barb to the Brain
A 27-year-old drunken Frenchman refused to divulge his identity to the police, who responded by shooting him in the head with a Taser (presumably because someone had mislabeled it "Lasso of Truth"). Later on, the man went to the hospital complaining of a terrible headache, as if the troll from Cat's Eye had crawled up his nostril to do Zumba in his cranium.
One of the barbed Taser darts was still embedded in the man's skull and had penetrated his brain, presumably activating a latent invisibility gene because nobody could see this without a fucking X-ray.