Some of the choices are interesting and appear to be based more upon the misuse that certain weapons were put to by their users, rather than necessarily any real problem with the design.
Examples from above are the Chauchat and the L85/SA80. The Chauchat in French service never provoked the condemnation it did in American service - interesting that. The L85/SA80 was in its earlier forms flawed but in its latest variant acclaimed as being more reliable than the weapons which it was trialled against (the M16a2).
The M16 design is flawed but not so much as to have perhaps prevented it seeing service. When modified with a gas piston, it is said to be excellent.
The only weapon off the top of my head which should never have seen service was the M60 GPMG. Unreliable and physically weak, its design was such that it was possible to place the gas piston into the cylinder backwards if one was not careful and that prevented the weapon functioning at all. Interestingly and perhaps ironically considering what I just typed, users had to purposefully install the clip which was designed to hold the retaining pins for the pistol grip and trigger group upside down because if installed the correct way up, it was prone to shaking itself loose.
The weapon was, because of the poor quality of the materials utilised, prone to a condition called "run away gun" where it would continue firing until the ammunition supply was exhausted once the trigger was released. In the Australian Army the approved IA (Immediate Action) for such an eventuality was to throw the belt of ammunition over the gun, so that it would purposefully cause a stoppage.
In 1960, when the Australian Army carried out trials for a replacement for the venerable and well liked Bren Gun, there were two contenders - the M60 and the FN-MAG58. The FN-MAG58 actually won but political considerations ensured that the M60 was adopted. When the M60 was finally replaced, guess what weapon replaced it in Australian Army service? The FN-MAG58 (actually the British L7 version).
In the late 1970s, the US Army carried out trials of their own for a new tank co-ax machine gun. The weapons trialled were an "improved" version of the M60, supposedly optimised for tank work and an FN-MAG58 also optimised for tank work. The M60 failed dismally. They found the only way they could get a stoppage in the FN-MAG58 after 100,000 rounds fired through it was to pour sand into it. After 500,000 rounds the FN-MAG58 showed minor cracking on the body. The M60 had failed after only 25,000 rounds, showing cracking already and by 100,000 rounds it was condemned as unsafe, IIRC.
The results were such that even the US Army couldn't ignore them and so they adopted the FN-MAG58 as a co-ax in the M1 tank. The Infantry quickly followed.