As I said earlier, I think in this case the sin that has been done is not the particular action, but the negligence of the person who broke that rule unintentionally--negligence being ungodly because it is a failure to responsibly manage oneself and often comes out of laziness. Once again, it is the mindset of the actor that is important. We differentiate between actions that are completely accidental and unforeseeable and those that could have been avoided if a person had been being responsible.
In the case of a complete accident involving the death of another, God set up the cities of refuge so the person who killed another could escape the penalty for murder. In the case of premeditated murder, the penalty was death. The case of negligent homicide was not addressed, which I believe means that the elders would debate it among themselves and decide the appropriate punishment.
Now if someone kills someone else completely accidentally, no charges are filed. If it is premeditated, you are tried for first degree murder. If it was due to negligence, you are tried for negligent homicide (I unfortunately know of two "friends of friends" who were tried for negligent homicide committed while driving a vehicle--drive carefully!)