It seems the rule of thumb is, if you feel fine immediately after a shot, like no ill effects within the first five minutes, whatever screw-ups happened aren't serious. I've been having trouble injecting in my right side lately. It's hurting a lot more and last week when I pulled out the needle blood gushed down my leg. I'd done a lot of reading and it's not too uncommon for the needle to nick a vein on the way in or out, so I didn't panic, though it was discomforting. If you aspirated and saw no blood you'll be fine.