I have written a Nix expression for a Python environment used in another Nix expression for a Docker image containing a collection of mostly-Python software I'm maintaining.

Most of the time the build succeeds.

Sometimes, not very often, the build fails with an error very much like this one:

rsync: readlink_stat("/nix/store/898mvvvdja9a8svkvnyid72gwggjfk92-python-2.7.13-env/lib/python2.7/site-packages/nevow/js/Nevow") failed: Too many levels of symbolic links (40)

So far as I can recall, it has always been an error about lib/python2.7/site-packages/nevow/js/Nevow (as opposed to some other file or directory in the big heap of files and directories going in to this thing).

The build environment where these successes and failures are observed is the numtide/nix-builder Docker image. Some builds are visible on travis-ci (but of course the information at this link is going to change over time). There's also a tool in the repository which should run the whole build process, making the failures repeatable in principle (ignoring the fact that they seem to be intermittent in nature).