Factual error: The bullet removed from Cole's leg is totally undamaged. Even a full metal jacket bullet will be squashed when it hits a person's leg (and stops inside the leg).
Factual error: In the beginning, James is collecting insects but it is cold enough for snow. Insects don't stay exposed to the cold like that.
Factual error: In the madhouse scene where Brad Pitt is jumping on beds and ripping up pillows, feathers are strewn about. Feather pillows are never used in secure facilities, for sanitary and allergic reasons.
Answer: The tattoo is for the movie. It is his prisoner identification.