Answer
The liberum veto, a parliamentary rule in the Polish-Lithuanian Commonwealth, allowed any single member of the Sejm (legislature) to nullify an entire bill or motion. Frequently abused during John Casimir's reign in the mid-1600s, this provision made enacting laws and promoting effective governance exceedingly difficult, contributing to the ongoing decline of the Sejm and exacerbating the Commonwealth's instability and gradual downfall.