In both cases, the analysis of the materials obtained from the scene of the suspected offense resulted in a charge unrelated to the offense initially suspected. When laboratory analysts determined that some of the evidence delivered to them proved to be explosive, the suspect in each case was charged with illegal possession of explosives. The circumstances of these two cases indicate that when police officers and lab analysts encounter unknown materials at crime scenes or in forensic/toxicological laboratories, more care should be taken in their handling. In one case, the unknown substance was a heavy metal azide (lead azide), which is dangerous to people’s health and life because its decomposition is difficult to control. The second case involved sodium azide, whose decomposition can be controlled without difficulty. The majority of chemical compounds in the category of explosives are stable, i.e., they are resistant to shock and mechanical factors, so the risk of explosion during handling or laboratory work is low; however, the forensic chemist must continuously guard against the tendency to treat in a perfunctory manner materials that look the same or that are presented as evidence of one type of suspected crime. In cases of unknown substances, preliminary screening methods should always be used in order to determine whether the unknown materials are a direct threat to health or life. In both of the described cases, infrared spectrometry should have preceded examination for a specific component by other analytical methods. 6 figures and 5 references