If a community instituted a pronouncement that only a specific butcher would be allowed to slaughter, and another butcher goes ahead and slaughters, (regarding what he slaughtered) There are those who say that what he slaughtered is forbidden.
If someone slaughters an animal for a non-Jew in the presence of witnesses, and a Jew comes to purchase it from him, and he says "do not buy it, because it wasn't slaughtered in a kosher manner", we do not believe him, [and the buyer is allowed to purchase it (*based on the testimony of the witnesses who saw it was slaughtered in a kosher manner)]. However, for the shochet himself, it is forbidden for him to eat it, since he declared it to be a forbidden item for himself (i.e. by his statement that it wasn't kosher).
If a butcher made a sign on the head of a (kosher) slaughtered sheep, in order that it should appear to be treifah, and also he said it is treifah, but subsequently he said it was kosher, and he only said that it wasn't kosher earlier in order that he could take it for himself therefore it should remain (unpurchased) so he could take the meat from it, since he gave a reason for his words, he is believed (that it is indeed kosher and only appears non-kosher based on a post-slaughter butchering technique).