Veins
I have no veins. I had a blood test recently at a doctor's appointment. The nurse pored over my arm for five minutes without attempting to stick me and called a more senior nurse. Together, they bound, pored over, and slapped my arm for a further fifteen minutes (admidst cheerful discussion about my veinless state) before finally getting one, luckily on the first go. I have a big bruise, but unlike when I was admitted to hospital for appendicitis, I wasn't stuck several times. They were nice nurses; they took great care not to stick me until they were as certain as they could be that they'd found a place to do it.
If I was on death row, I somehow doubt the people administering the injection would be so careful. Which just adds a little frisson of horror for me as I read that story.