Within a parallel build, modules A and B can start at the same time and decide at the same time that they need to download artifact X. Since X is not available locally, both can start download of X. Both will download X and both will try to write the file for X to disk.
On Windows platforms, one of them can fail with file locking problems. On Unixes the contents of the written file can be non-deterministic.
This issue is related to
MNG-2802 in the sense that it can create a corrupted artifact file in the local repository, and it can happen to ANY file written. But as opposed to MNG-2802 the synhronization needed to fix this issue is basically only within a single running maven process, so regular java mechanisms can apply, whereas MNG-2802 probably needs to use a totally different solution space.