This sort of thing came up for me on two successive occasions lately. The six months right before that had zero instances at all.
This morning during my usual shot of 50 mg NAD I must have nicked a vein. The needle met more than ordinary flesh and a coin-sized spot went dark purple right away. I stuck to my normal steps yet came out unlucky. Nothing hurt and no strange reactions showed up. All I got was a big mark from the bruise plus that nasty sensation from catching the vessel.