I found Baker's dog food in a 3kg bag of Purina One not so long ago....I both phoned and emailed them, the phone no asked for the batch no of the bag and where Id bought it, email reply said they would send a bag for a 200g sample to be sent back to them!
But of course I ignored the email request....200g sample indeed!
Them on the phone were brill, just posted back about 5 bits and after approx a 2 weeks got £15 worth of vouchers and an apology stating that the machine that packs the cat bics was used at an earlier date for the dog bics, usually they take off the first 5 bags or so as to not have any cross contamination....
Since then Ive had about 4 more bags with them in.....I never bothered getting in touch though!