Yep. Has happened to me. After swabbing your stopper, let it dry completely and see what is left on it. You would be surprised, you can find non cotton alcohol pads and that helps. If it were a whole vial, I'd say filter it with a whatman syringe filter. But I would bet money you pushed some cotton strands through the stopper.Could it be possible that its from me drawing? Maybe fragments from the alcohol swab on the rubber stopper? Just thinking of all possibilities.
I say this because as you see I've been pinning it and I've had no issues