Definitely warmth and gravity for me too - and I certainly find it harder in winter than summer.
I had a new one for me last weekend. I reckon I've done something like 35,000 finger pricks and for the first time, must have hit a little artery in my little finger. As I fired the lancet, a jet of blood shot into my eye and covered my face and front with a mist of tiny red droplets. I did let out a wee astonished noise.
A few years ago my sister had an injection in her bum at the docs and he did something similar, hitting an artery, sending a reasonably substantial jet of blood over the carpet and up the wall opposite. The doc was making noises about the cost of cleaning the carpet and hinting my sister would be getting the bill to which she replied "you were the one steering the needle, not me".