Production of a trill depends on several articulatory and aerodynamic constraints. These constraints can be held responsible for various sound changes in Slavic languages which all involve depalatalization or frication of Proto-Slavic palatalized trilled r. As soon as a trill is affected by palatalization, the aerodynamic conditions are changed and the possibility of trill production lowers. Small deviations in aperture size and air velocity can lead to a failure of a trill. This paper proposes a phonetic explanation for the depalatalization and/or frication of the Proto-Slavic palatalized trilled r by considering the detrimental effects of articulatory and aerodynamic constraints on the production of a palatalized trill.