The injection is hurting/burns or the plunger is giving me a lot of resistance (requires a lot more pressure than usual to press down). Either way, despite seeing the blood and thinking "WTF, I registered!!," it means you are NOT in a vein and should abort.
I had this happen last night! I'm usually pretty proficient with a 5mm needle but the pharmacist gave me the half inchers and the effect has been devastating. My vein are close to the surface and rather easy to hit, this long needle ended up ripping my veins up so badly. I would try to insert all the way because it's so easy for the needle to slip in/out if you don't get it in there all the way. So I'd come in at a VERY slight angle and end up going over top of the vein (and it hurt like fuck!!) OR going right through. If I actually made it in there, I'd be so nervous that I would shake and twist the needle around in there and attempt to register many times because of all the shaking.
Anyway, I was on my last clean rig of the night after going a little wild with IV MPDV and was attempting to hit a vein smack in the center of my forearm. Went in and disliked the angle so I pulled out and tried again further up with better success. Pulled back on the plunger and got a burst of air and some blood, I started shaking and my hand were sweaty as hell (maybe from the PV) and I couldn't pull back the plunger to try and register again. I guess I finally did register and slipped the needle around as I tried to grip it to press down on it.
When I started to press down, the plunger was offering a lot of resistance. It also seemed like the needle was sliding itself out as I tried to press the plunger. I decided to abort, unsure of what had happened but glad I didn't just slam the plunger down.
Immediateley I could see a bump form. Within a few minutes it got painful to press on, and then an angry purple reddish bruise could be seen running along the vein with some wider swelling around the injection site. I definitely spent some time panicking about abscesses and other complications but today it's just bruised and only raised ever so slightly so I think it's going to be okay.
I absolutely cannot use the longer needles, I have bruises everywhere on both arms, I skewered the vein in the crook of my arm by poking the damn thing 4 times, I have little patches of red blood in many locations, my arms have always been pristine but after two days of these needles they are a fucking warzone. I guess some people can use these just fine but they're just not right for me.
I ended up reusing one of my short needles that has been used easily 4 or 5 times and it went in, I registered and shot and it didn't even leave a mark. I know that I don't need to put the needle all the way in with the long ones to hit the vein, but being able to slide the needle around is distressing to me as I fear I will miss the shot.