The Chauchat (pronounced 'show-shaw') was the primary French light machine gun of WWI, first introduced in 1907. It is generally regarded as one of the great engineering failures in the history of firearms. It fired 8mm rifle rounds at a relatively slow rate of 250rpm. However, it is estimated that over 250,000 were manufactured, making it the most widely manufactured weapon during World War I.
Among its many problems were the fact that it only accepted 20 round box magazines, which required reloading after only a few seconds of sustained fire, as compared to the Maxim gun used by virtually every other combatant in the war, which used a belt of 500 bullets (or more). The reason for the supremacy of the machine gun on the WWI battlefield was the ability to lay down a constant hail of bullets against the inevitable human wave attacks. Reloading constantly obviously undermined this tactical ability.
Furthermore, the Chauchat was designed in such a way that mud and debris easily found its way into the inner workings of the gun. Considering the muck ridden environment of trench warfare this was an invitation to disaster. It jammed constantly even when clean, though this only heightened the already existing problem of having to stop firing frequently to reload anyway. The most frequent malfunction was failure to extract spent shells, and thus the next round would get wedged into an occupied chamber, siezing up the mechanism. Additionally it used a 'long recoil' system that threw a tremendous force against the user's shoulder, resulting in a very unweildy weapon, even when it worked correctly. It was of a rather shoddy stamped metal construction and has sometimes been referred to as 'the worst machine gun ever issued to any army at any time in history'.
Due to political reasons, American Expeditionary Force fighting in WWI were issued Chauchats from their French allies, rather than their own far superior Browning Automatic Rifles. They purchased 34,000 in 1917, but when the Americans realized how deficient the Chauchat was, legend has it they disasembled the guns and used the parts for mundane tasks like tent poles, and relied on their bolt action rifles instead for actual fighting.