When is a mistake of fact a valid defense?

Many crimes require the defendant to have a certain state of mind in order to be guilty of the crime. For example, murder requires the intent to kill someone and larceny requires the intent to permanently deprive someone of their property. Imagine if Bob shoots and kills Sam, then hops on Sam's bike and rides it away. Seems like a murder and larceny, right? Let's introduce more facts: At the time of the shooting, Bob was deer hunting and Sam was negligently playing in the forest wearing a deer costume. Bob believes Sam was a deer when he shot and killed him, thus this mistake of fact means that Bob lacks the state of mind to be guilty of murder. Similarly, Sam and Bob have the same bike. Though Bob did take Sam's bike, he thought it was his; this mistake of fact means that Bob lacked the state of mind to be guilty of larceny.