We've just had OpenSIPS 1.4.2 stop processing SIP packets and effectively hang. During this time, it logged the following many times to /var/log/daemon.log:
ERROR:auth:build_auth_hf: no more nonces can be generated
ERROR:auth:challenge: failed to generate nonce
Restarting OpenSIPS has temporarily cured it, but I expect the problem will come back.