Planning is a great way to reduce the risk of unexpected outcomes.  That is your action is misguided and the results are wrong.

Judging the occurance of mistakes can be a bit subjective, one person might think a mistake has been mase while another does not.

Inevitable

While the frequency of mistakes is reduced by planning it is never eliminated.  While we generally work to avoid the occurance of mstakes, they are not necessarily bad.

Overcoming an undesirable result can leave to improvement and of course experience and life learning are invaluable.

Identify, Fix, Move on

Most importantly do not deny or ignore the outcomes when a mistake occurs.  It might be appropriate to only acknowledge what happened with no further action required, the key is this needs to be a conscious decision.

Of course a correction or improvement is often the best course.  This is really just a replanning exercise.  Consider what occured, the root causes and the best next steps.

It is an Opportunity

I almost always gag when a boss or colleague suggests a problem or issue is really just an opportunity.

Applying this logic to mistakes (which can be a sub-set of problems or issues) works for me as typically they are of your own making, you own them.

A mistake affects you and is related to your involvement.  It is not unusual to expect you to act.  Viewing the mistake as an opportunity rather than a defeat, will help you do that.