This dissertation object study is contra legem decisions, especially those that don t keep the procedural rule. It is understood by contra legem decision the act by which the decision maker faced with a peculiar situation decides not to apply the legal rule - the law - usually because has the understand that this application potentially violate constitutional principles. For this reason, studying norms, rules and principles is essential to understand the topic, mainly because they can set antinomies between normative conflict situations, which through the technique of weighting can lead to contra legem decision. Once the rules are casuistically apart, a deep review about constitution is required, including specific topics such as their rationale (justification), its antecedent and consequent. The principle of due process of law and legal certainty are grounds of procedural rules, and because of that when a law process is passed by, they are as well, indicating that they should be considered by the judge to decide contrary to the law. Finally, exposure of specific cases enables the application of theoretical concepts in real situations, which facilitates the understanding of the position of the courts on the issue of contra legem decisions.