Anna Dot takes the popular expression “stumbling twice over the same stone”. Understanding that the word “stone” is a synonym of “error”, she asks herself what shape errors could have. Would it be possible to convert errors into visible forms so that they can be recognized and not be repeated again? Would we then trip twice over the same stone? Dot develops a mathematic system to obtain the shapes based on Don Quixote’s errors of perception in Cervantes’ novel.