This paper aims to provide a notification related to the inner filter effect (IFE) that can affect the accuracy of measurements in fluorescent spectroscopy. This effect can cause the measured fluorescence to be distorted than it should be, leading to inaccurate interpretation of experimental results. Especially, this can become more pronounced when the substances are highly concentrated. Here, carbon dots (CDs) were experimentally employed as an illustration to this effect when measuring photoluminescence (PL). Contour plots of the PL emission were employed to visualize spectral changes under different optical densities and measurement configurations. A pronounced discrepancy was observed between the high- and low-concentration samples, demonstrating the role of IFE in fluorescence results. These findings serve as a cautionary reference for researchers working with fluorescent nanomaterials, emphasizing the need to control experimental parameters and validate data interpretation, particularly in PL studies of CDs.