The needle broke somehow, contents of the injection spilling all down my husband's leg. Instead of our baby passing quietly her last 30 seconds were spent squealing in pain.
I know we could cope better with the loss if it hadn't ended this way. I don't even know how to make peace in my mind with the way things ended. Somehow we'll have to but we're devastated.
I'm sorry for your loss.

I can sympathize.
I've been there for three of my dogs being PTS. With two of them, the drug was injected in without prior sedation and there was some squirming and such. It breaks my heart to think about it.
The last time I had my dog PTS, I had a different vet who did it so well, I'd never do it any other way again. She gave my dog an intramuscular injection prior to the euthanization which sedated him as if he was having surgery. Instead of the stress of holding him down and him dying right there, he fell asleep on the table and we had a nice conversation about his life while patting him gently. Once he was fully "under" and I was "ready" (and she asked me whether I wanted to stay for that part or not, which I did but I went over and looked out the window) she gave him the final intravenous injections (three, actually - the first one infiltrated, and the second one was not enough, necessitating a third

thankfully he was sedated beforehand or this had the potential to be a nightmare). A much better way than injecting directly while the pet is awake. I sincerly hope anyone reading this who has to go through this difficult process considers requesting this method if it isn't offered.
My suspicion as far as the needle goes is that the locking part (needle to syringe, which are separate) became unlocked and separated, spilling the medication, as opposed to the needle actually breaking. I am a hospital nurse and use needles like this to draw up medication, mix drips, give shots, etc, and it happens a fair amount, spilling medication on
me (I'm sure many nurses can attest to this). Obviously, not good in the setting of an actual euthanization, but at least I would think that your pet did not feel pain as a result of it; it probably, however, delayed the process and caused a bit of chaos (in trying to reconnect the needle to syringe and/or possibly having to redraw up more medication) in the heat of the moment. Hope this gives you some peace about it.

(If you are unsure you could always ask the vet who performed the procedure to explain exactly what happened as it is bothering you. I would think he or she would understand.)