I have been giving my 25 year old Polish Arabian mare penicillin shots for the past four days for a staph infection, and I had been doing quite well. Tonight I went to give her the shot and drew back to ensure I wasn't in a vein. I saw no blood so I proceeded to inject the penicillin. She suddenly brought her head up, her eyes glazed over,and I immediately removed the needle. In the next few seconds she collapsed and began thrashing around. She tore herself up really bad and the right side of her face is swollen as she hit it on the barn door frame. I called my vet and he told me I must have hit a vein. I don't understand though: I drew back and saw no blood. I guess the needle slipped or something. I'm just very upset that my elderly mare is now torn up and feeling miserable. I feel even worse because I am hoping to be an equine vet, and I screwed up badly. Now I wonder if I should even continue pursing this goal as I am now afraid to give shots. Is it possible that the needle slipped somehow after the fact? I'm hoping somebody has more expereince with penicillin and can enlighten me. This scared me really badly, and I thought my dear Ariel was going to die.

Your absolutely correct that is extremely scary to watch.
Luckily it was apparently a small amount & she recovered.
You don't say but if you did not rotate your injection site/side you could have hit the same spot that had already been injected multiple times and got a false read on your needle, or it could have shifted as you think.
Don't think your the only one this has happened to, my Vet gave a bad shot once, and he's already graduated Vet school decades ago.
Give her big hugs and ask if you can continue her treatment w/ an oral.