I deal with chicken every day and I have never seen anything like this, there was a small dot of dark color in the breast of this chicken and I kinda picked at it, trying to remove it. Ended up digging under the skin and finding this large chunk of hard discolored stuff. No idea what it was but I threw that entire chicken away raw, no way I was risking selling that to a customer if I didn't know what was wrong with it!
