I remember the very first time I injected I was scared out of my mind. And then, when I pulled out the needle, blood went shooting out everywhere. I started freaking out and thinking I had seriously f*cked up. But, I had just hit a vein and for some reason the blood came squirting out of the injection hole. I put a cotton ball covered with hydrogen peroxide on it and held it there and it finally quit
Quite a scary experience and quite a mess to clean up.
So yeah, it happens. Nothing to worry about, just make sure you're pulling back on the plunger once you reach the injection site and aspirate and check for blood. If there's no blood and just bubbles, you're good to go. If you pull back and see blood, pull the needle out and try another injection site. Hitting a vein or artery can be deadly if you inject it with oil.