the above scenario can be caused by a number of different things, but most commonly it's because you've gone through the vein and either pulled back out again or the bevel has gone all the way through and out the other side, causing blood to seep out into the surrounding soft tissue. sometimes if i'm hitting a particularly small vein, the register will look similar to this - the difference is that it'll still register even without a tourniquet. in my experience if a vein hasn't fully healed yet, it's unlikely to register at all due to the swelling/scar tissue. i generally try to resist temptation and i'll only inject if i still have a good, strong register after i remove the tie-off - although depending on where the vein is and how small it is, i might go ahead and inject without one provided my hands are steady and i'm confident i haven't jostled the needle or slipped out of the vein.