bubble always there, and from what i understand it takes quite a bit of air getting directly into your vein before there is any harm done.
Once, on a glute inj, i aspirated and drew blood, when i went back in, i had not gotten out air from pin, there was 1/4 cc in there about. I knew it when i heard the bubbly sound at end of inj. I could feel air bubble going up my back later. Never caused another prob.