I think this is a valid reason to complain. For one, you didn't get what you paid for. Two you could have been injured while drinking it. Additionally, I feel people complain about far less situations(ie/writing to a company about expired coupons on a box of cereal)when you knew when you bought it they were expired and demand replacement for them! Really? IMO just cut your loses and don't cry about the few $ you may have spent on the cereal. Afterall the cereal is still good. Having plastic in your cup is not good at all! Especially if it had been a child drinking it!

