rsync error: failed to set times on "/foo/bar": Operation not permitted

The issue is probably due to /foo/bar not being owned by the writing process on a remote darwin (OS X) system. A solution to the issue is to set adequate owner on the remote site.

Since this answer has been voted, and therefore has been hopefully useful to someone, I'm extending it to make it clearer.

The reason why this happens is that rsync is probably trying to set an arbitrary modification time (mtime) when copying files.

In order to do this darwin's system utime() function requires that the writing process effective uid is either the same as the file uid or super user's one, see opengroup utime's page. Check this discussion on rsync mailing list as reference.

If /foo/bar is on NFS (or possibly some FUSE filesystem), that might be the problem.

Either way, adding -O / --omit-dir-times to your command line will avoid it trying to set modification times on directories.