Sometimes people don't stick the pin high enough on the thigh and you have more of a chance of hitting a vein. I haven't had anything bad happen except for some blood coming out after an inject. I aspirated and everything and saw no blood but when i pulled the pin out blood just started flowing. stopped easily enough just was surprised because it never happened before. i'm worried about hitting a nerve....don't know what i'd do if my leg started flopping around.