I recently ran into a Maven problem in which standard artifacts that were hosted on Central could not be downloaded. I eventually traced the problem to the use of a mirror. Our team had been using a local installation of Nexus to mirror Central, as described here:
The problem was that the mirror was down. Now, I knew the mirror was down, but I assumed (wrongly) that Maven would try to route around the problem by skipping the downed mirror and obtaining the artifact directly from Central. (It is, after all, only a mirror, not the actual repository.) Instead, if a mirror is down, Maven simply aborts, saying it cannot download the file. This led me to the mistaken conclusion that Central itself was down.
It would be nice if this problem could be fixed so that a downed mirror doesn't cause Maven to fail, but that might take a lot of work, so in the meantime I'd be happy if the problem were simply documented. Unfortunately, the "Guide to Mirror Settings" in the link above talks a lot about how to declare a mirror, but it doesn't say what Maven does if it can't connect to a mirror. It should be amended to include this information.