False belief, as an excuse, attributes the action to a false belief.
In general, an excuse is a claim that blocks the inference from some prima facie wrong action to the conclusion that an agent acted in a way different from how a person with good desires would have acted.
For example, a traveler at an airport picks up a suitcase and walks away with it. The luggage belongs to somebody else. We may well imagine that some thief is grabbing luggage and taking it. However, our traveler in this case took a piece of luggage that he honestly thought was his. The rightful owner of the suitcase discovers, after all the other passengers had left, that there is one suitcase remaining that looks like hers, but it is not hers.
The traveler would respond to the accusation that he took property that did not belong to him by claiming that he made an honest mistake. He thought the suitcase was his.
Desirism holds that morality is primarily concerned with evaluating desires. However, our intentional actions are not a result of desires alone. Desires interact with beliefs to form intentional action. People choose those intentional actions that would objectively satisfy the most and strongest of their desires in a universe where their beliefs are true. When their beliefs are not true, they risk performing actions that fail to fulfill their own beliefs and desires.
Our traveler in this case performed an action that would have objectively satisfied his desires in a universe in which the suitcase he picked up was actually his suitcase. He is not showing the absence of an aversion to taking the property of another - an aversion that people generally have many and strong reasons to promote. He wanted to take his own suitcase. False beliefs thwarted his intentions. Now he has to spend time taking this suitcase back to the airport and finding his own.
However, can we really count this as an excuse?
After all, if a person has an aversion to taking the property of another, and they can reasonably predict a situation will arise where they could take the wrong suitcase, the responsible person would take pains to make sure he gets the right suitcase. He checks the name tag, or attaches some identifying object to the bag to help to guarantee that it is his own, or both.
If a traveler does not do this, and ends up with a bag other than his own, we may still condemn him for his carelessness. We may condemn him for not caring enough to avoid the situation where he walked out with the wrong suitcase. "Be more careful next time," the owner snarls, justifiably so, when she gets her own luggage back.
"I didn't know the gun was loaded," is an attempt to use the excuse of false belief. However, this excuse fails. Even if it is a sincere mistake, the possibility of being wrong can do such great harm, and the possibility of doing good is so small, we are morally required to assume that a gun is always loaded. In these situations, "false belief" never becomes a legitimate excuse, because a person with good desires would never create such a risk. Rather than provide a defense against condemnation, a "false belief" that is poorly grounded or that demonstrates a disregard for the interests of others can itself be condemned as an instance of epistemic negligence.
The potential harm of false belief implies an obligation that care be taken to avoid harms. The greater the harms that could result, the stronger the obligation to ensure that one’s beliefs are well founded. If the harms are great - if, for example, the situation is one where false beliefs could result in failure to prevent widespread global destruction of land, property, and health, the people with the false beliefs may still be properly condemned - and a person's defense of his belief shows carelessness, she may be soundly condemned.
This would be a case in which an agent's behavior shows that she did not care enough about this potential harm to make sure that their beliefs about this harm were properly secured.
False belief is not sufficient to excuse an agent from wrongdoing. The false belief must not be an irresponsible belief. An irresponsible belief also shows lack of an aversion to the harmful states that a false belief could create.
However, there is certainly a realm in which people make honest mistakes. Sometimes other people pay for our mistakes. If the false belief was responsibly acquired, the person can honestly say that, “My behavior is grounded on a false belief that even a responsible person would have had.” When this is true, false belief can be used as an excuse, the behavior does not imply that the agent had desires that people generally have reason to inhibit through condemnation, and condemnation would not be appropriate.