Someone who does something wrong or against the rules for a long period of time and then quits and tells on others doing the same thing.
Adam: "Dude, both me and Dave took off 10 sick days last month! This month hes decided to be good about it and now hes ratting on me for calling in sick too much!"
Josh:"Wow dude, hes just being a Canseco right now. He got what he wanted out of it and now hes the informant"