Bechorot 35: Intentionally Creating a Blemish,

A blemished firstborn kosher animal was sacrificed to the Temple until the Temple was destroyed.  At that point it was given to the priest who would wait until the animal had a ba'al mum, a permanent blemish.  The priest was then allowed to eat the animal.  Today's Mishna teaches that a priest might deliberately blemish the animal.  Suspicion of such behaviour would lead to the Sages forbidding that priest from slaughtering the animal.  

We are told that a Roman official asked about the status of an old ram whose hair was long.  They replied that it is a firstborn and it may be slaughtered only if it has a blemish.  The Roman used a dagger to cut the animal's ear.  The Sages permitted this animal to be sacrificed. The Roman then hurt many other firstborns.  The Sages changed their ruling.  The Mishna concludes with a rule: whenever the blemish is caused intentionally, it is forbidden.  If the blemish is not intentional, it is permitted.

The Gemara uses cases to illustrate this ruling.  Rashi says that the Roman officer was hurting animals just to taunt the Jews.  When they learned he wanted to permit the animals, the Sages forbade it.  In Commentary to the Mishna, Rambam says that the the Roman officer knew that his behaviour would permit the animal, but the Sages permitted it because he was not told to do this by the Jewish owner.  After the Roman official did this many times and there was no negative consequence, the Sages decided to forbid the animal.  The Jews were giving the impression that they had asked him to injure their animals.