Can you sin without knowing it?

[toddy]

Because devils deceive it is no sin to do such thing. They are very smart, 1000+ years experiences in cheating, work 24/365, can read minds and know what you want or think…

 

[Genesis315]

Short answer is yes, if one should have known and one was at fault for being ignorance, then guilt can be imputed to acts done in ignorance. From the Catechism:

1791 This ignorance can often be imputed to personal responsibility. This is the case when a man "takes little trouble to find out what is true and good, or when conscience is by degrees almost blinded through the habit of committing sin."59 In such cases, the person is culpable for the evil he commits.

Also, it bears pointing out, "no one is deemed to be ignorant of the principles of the moral law, which are written in the conscience of every man. " (CCC 1860).

Note, the “principles of the moral law” are not the entire moral law–they do not include “conclusions” which require additional inference from these principles. Here’s how the Catholic Encyclopedia sums this up (note, the “primary principle” is simply do good and avoid evil):

Passing from the primary principle to the subordinate principles and conclusions, moralists divide these into two classes: (1) those dictates of reason which flow so directly from the primary principle that they hold in practical reason the same place as evident propositions in the speculative sphere, or are at least easily deducible from the primary principle. Such, for instance, are “Adore God”; “Honour your parents”; “Do not steal”; (2) those other conclusions and precepts which are reached only through a more or less complex course of inference.
CATHOLIC ENCYCLOPEDIA: Natural Law