Over at Marginal Revolution, Tyler Cowen posts about the liberum veto, an institutional innovation of the Polish-Lithuanian Republic, which began sometime in the 14th century and lasted until Poland was partitioned by Russia, Prussia, and Austria-Hungary in the 1790s. The liberum veto allowed a single member of Poland's parliament, the Sejm, to block legislation and, eventually, to nullify all legislation previously adopted and adjourn the entire session of parliament simply by saying "Nie pozwalam" ("I do not allow it").
Tyler doesn't really comment on the institution of the liberum veto, but many commentators to his blog added color and content. As I have written extensively about Polish constitutional history, including the liberum veto, in the past, I can add some further information about it.
In contrast to what many, particularly Western European, historians have assumed, the liberum veto was not an inherently destructive institution that disabled governance in the Polish-Lithuanian Republic. In fact, the institution existed while the Polish-Lithuanian Republic was growing into Europe's largest country - at its greatest extent ranging from the Baltic Sea to the Black Sea - allegedly without fighting a single war of aggression. The map below shows Poland at its largest, after union with Lithuania.
Between the 14th and 16th centuries, Poland developed a system of elected (non-hereditary) monarchy, which was subordinate to a sizable nobility - Europe's largest as a percentage of total population. In terms of political rights, including voting rights, every member of the nobility (known as the szlachta) was equal, regardless of great differences in wealth. Every noble was entitled to sit as a delegate to Poland's parliament (the Sejm), and to vote on the election of new kings.
Among the many novel institutions of Poland's Sejm was the liberum veto, which for centuries was used mainly as a threat - something like a modern filibuster - to force parliamentary leaders to forge compromise solutions to avoid a deadlock. In addition, the use of the liberum veto was limited by other institutional features of Polish proto-democracy, namely the system by which delegates to the Sejm were selected and instructed. Delegates were sent by local or regional sejmiki (little parliaments) with specific instructions on how they were to vote on issues to be addressed by the Sejm. If they deviated from their instructions, the locality or region they represented was held not to be bound by the Sejm's decision. Because delegates were bound by their instructions, they could not simply threaten a liberum veto willy-nilly. They had to comply with their instructions.
It was only in the 17th century that the liberum veto turned into what the Polish legal historian Wenceslas Wagner labeled a liberum rumpo, which served to dissolve the entire Sejm. At about that same time, foreign powers began to buy off the great Polish magnates to interfere with Polish parliamentary processes, which led to a weakening of the Polish state and armed forces, to the benefit of those foreign powers, which ultimately partitioned Poland. The Polish state disappeared from the map of Europe from the end of the 18th century until 1920.