Mishna: One who breaks the bone of a Paschal lamb that is ritually pure receives forty lashes for having violated a prohibition stated in the Torah. But one who leaves over part of a ritually pure Paschal lamb and one who breaks the bone of a ritually impure Paschal lamb do not receive forty lashes.
We see from here that a person will only be held liable for this transgression if the korban was kosher. If the korban became tameh (ritually defiled) and could not be eaten, then the breaking of a bone would not lead to lashes.
Many suggestions are made as to the reason behind the prohibition against breaking bones in the Passover sacrifice. One approach is that the korban Pesah must be eaten in the manner of the “upper class” – free people who are not ravenous, nor even hungry enough to go to the trouble of sucking marrow from the bones of the meat.
Some commentaries ask how we know that there is a prohibition against breaking bones in the korban Pesah. Perhaps the passage that says “and a bone will not be broken in it” simply means that it is not necessary to break the bones in order to extract the marrow – something that we may have thought essential, given that it is prohibited to leave edible meat from the korban for the next day (Shmot 12:10). One answer is the Torah’s emphasis on the word bo – that a bone cannot be broken in it, in the korban Pesah – only makes sense if we are talking about a commandment
The Jerusalem Talmud adds details to the discussion that takes place in our Gemara on the topic of breaking bones. According to the Yerushalmi, for example, there is a separate prohibition on each bone that is broken. There is also a discussion there on how large the break needs to be, a crack large enough to be felt by a fingernail that catches in it, or a larger break that can be distinguished by the touch of a hand.