I received the "you have used this code the maximum number of times..." message. So, I checked with live help. She told me "..hun, that's because you've already redeemed a free chip today". But told me to try it tomorrow.
So, that may explain why, at times, I got that message for a chip I had never used before. You all may already know this, but it was news to me; so, I thought I'd share.