The Taz YD 116:6 on the last line brings a case that he heard that one should protest putting a needle into bread. What is the Taz referring, to and what exactly is he worried about?
It seems that the Taz is saying that one should object to feeding a vicious dog a loaf of bread with a needle in it, as a way of disposing of it. Rather, the proper way to kill the dog is with poison. It seems that, according to the Taz, giving the dog a loaf of bread with a needle in it is not, in this instance, a violation of tzaar baalei chaim nor of bal tashchis. However, it is a concern that this will drive the vicious dog mad, and it can be a life-threatening danger to others.
Also, it seems that even some people nowadays think to do this. Consider this case, from Feb 2012:
A Michigan man who hid needles in meat that was eaten by a neighbor’s dog faces sentencing after pleading no contest to attempted animal killing or torture.