Bechorot 35 - Unwitting Intentional Blemishes
One Roman official saw an old firstborn ram with very long hair. When he found out that Jews could not do anything with an unblemished firstborn, he notched its ear with a spear, and the Sages allowed to slaughter the animal. He did this to other firstborns, but now the Sages forbade to slaughter.
Once kids tied the tails of lambs together, and caused a blemish in a firstborn, and the Sages allowed to slaughter it. The kids continued doing this, but the Sages forbade further slaughter.
If a firstborn was pursuing someone, and he kicked it and inflicted a blemish, they may slaughter it on the basis of this blemish. If he came over to kick it later, this blemish is prohibited. Now, this is obvious!? You might have said that he is releasing his pent-up anger, and this is a valid blemish, so the rule is that it's not. Some teach the opposite: if he became angry later and kicked it, this blemish permits the slaughter.
When an expert finds a blemish that could have been inflicted by a man, he needs a witness to testify that is was accidental, but people with vested interest cannot testify.
Art: Victor-Gabriel Gilbert - Boys Playing