from experience, it only happens when you get a little directly in your vein... i have had it happen a couple times, and those instances when i pulled out the pin there was a good amount of blood; with others' reports and experience im led to believe sometimes a little oil can get in this way... dont forget to aspirate... ive pulled back on quad injects a few times and been IN a vein... would probably be pretty nasty to put a mL of tren in one...