According to Art. 7 of the Code of Criminal Procedure, the organs of the procedure form their conviction on the basis of all the evidence, assessed freely, taking into account the principles of correct reasoning and indications of knowledge and life experience. The Polish criminal trial allows the above article to be applied to a conviction based on the testimony of the only witness in the case, which may lead to abuses, both on the part of the authorities of the procedure and the witnesses themselves. Importantly, making a false statement does not have to be a deliberate act. Inadvertent submission of false statements is not associated with criminal liability, and undetected - may pose a serious threat to the person whose testimony relates. The author of the article focuses on the aspect of inadvertent false testimony by potential witnesses of the event. The aim of this article is to illustrate the threats posed by the failure to apply the Roman principle of testis unus testis nullus (one witness, no witness) by the organs of procedure and judiciary authorities in Poland, which is not binding in the Polish criminal procedure (one witness, no witness), while applying the principle of free evaluation of evidence and essential failure to penalize the act of unintentionally giving false testimony. The article focuses on presenting the results of research conducted in the form of a questionnaire, among a group of 50 anonymous volunteers, and related to potential participation in events in which witness testimonies may be of key importance for the determination of liability or determining the course of events. The results of the research, in combination with the cited literature, allow, in the author’s opinion, to stir the belief that the testimonies of the only eyewitness are reliable and that they are useful in criminal proceedings. In the author’s opinion, inadvertent false testimony should be punishable on a par with deliberate false testimony. On the other hand, admitting evidence from the testimony of the sole witness in a criminal case should be more rigorous than the principle of the free assessment of evidence, which in turn would undermine the fundamental principles of applying the institution of a crown witness or the so-called a small crown witness and would require reform of the criminal law system.