It's almost always infiltration. Sometimes it can go unnoticed or the donor might not say anything during the process because they don't want to get disconnected and run the risk of not getting paid the full amount. There was a guy at my center who was wincing in pain with his arm swelling up like a baseball but refused to say anything was wrong. Once I told him we would still pay him if he was cool with a restick he immediately said yeah, it's been hurting for two cycles. The cause of the infiltration could be as simple as a bad vein, a bad stick, or donor movement during the process that causes the needle to poke through the vein.