Today's Moral Debate: Postdated sins?

So, if one considers sins to be breaking of rules or strictures that are closely linked to feelings of guilt...does it matter if after one commits a sin, one finds out that a rule wasn't broken after all?  Are they crimes of intent or action?

For example, if one snitches a donut from the conference room, and then later finds out that they were available to all employees, does that expiate the sin?  After all, at the time the person snitched the donut, they considered themselves to be stealing, and went ahead and did it anyway.  Should the guilt remain?  After all, no rule was actually broken.

It's something to consider over a cup of coffee and a donut.