I think we've all nicked a vein once or twice. No big deal. The only thing is that when you nick a vein, the needle is up against the vein keeping the cut closed until you pull out. Once you do that, two things can happen, 1: You'll have a slight bleed.
2. The blood can pool inside causing issues.
So you should always inject at a slow steady pace. I never understood why I see guys recommending just pierce it fast??!! Slow and steady gives a chance for the veins to move aside and less chances of hitting them.