Any one can make a mistake. Provided it is reported or, if picked up by the FOQA system, discussed honestly with the designated representatives, there is nothing to fear and every one will learn. Depending on the nature of the mistake, some extra sim training may be appropriate for the individuals concerned or, if there are wider implications, new information disseminated, included in training routines, and/or manuals and procedures may be changed for everyone's benefit. But as Check Airman has intimated, lying about it cannot be tolerated.