Hard to say what happened in this case. If I understand you correctly if one part of the link was missing it could very well be it wasn't installed properly as opposed to actually breaking where part of the broken side might still be attached and if it was new even less likely that it actually broke. I've used a few of these putting 9-10k miles on each quicklink on different chains and never had any problems or failures or don't know of anyone else who has had a failure. I have seen people who have trouble understanding how these fit together or don't know how to check if it's locked into place correctly. I've seen even more people have trouble trying to install the rivets that come with Shimano chains but both are reliable if done properly. This is one incident where you don't know for sure all the facts of what happened and should not scare you off from using KMC chains or their quicklinks. They're just as good as any Shimano chain.