An act’s harmfulness to public finances is graded. The legislator does not specify this harmfulness in greater detail and only relies on an example of a list of premises which should be taken into account when determining its degree. At the same time, the legislator decided that only a marginal degree of harmfulness of an act is a basis to refuse to initiate or to discontinue proceedings in a given case. Stating that the degree of harmfulness of an act is not significant, is significant or gross obliges authorities adjudicating in cases for breaches of public finance discipline to draw specific consequences against the infringer. The author focuses here on a discussion of vagueness of the “degree of an act’s harmfulness to public finances” and refers to the premises of assessment of this degree as well as problems that arise in this context at the stage of application of law in the decision-issuing practice. It was deemed necessary to mostly rely on the practical meaning of the concepts discussed, which affected the study’s methodology. The author intends to point out potential problems when interpreting under-defined terms in the Act on liability for breaching the public finance discipline, to determine the meaning of “an act’s harmfulness to public finances” in the pursuit of values encoded in regulations of this act and to demonstrate practical application of the measure in question. The study’s methodology involves mainly an analysis of universal legislation and judicial decisions of the Chief Adjudicating Committee in matters of breaches of public finance discipline and of first instance committees. It was also necessary to analyse the relevant law in force and commentary on it, limited to the views of domestic legal scholars.