From 11294.0.0 (at least so far) there was a failure with this in the logs:

Failed to set up user: failed to add user: useradd: /etc/shadow.21: lock file already used#012useradd: cannot lock /etc/shadow; try again later.

Full logs are here:
https://stainless.corp.google.com/browse/chromeos-autotest-results/260865974-chromeos-test/

This is probably the first time I've seen this, not entirely sure.