Using the HTTP server under Linux, the evhttp_bind_socket() function with a host parameter of :: will by default on many distributions make the socket dual-stack, meaning that a subsequent attempt to bind() to 0.0.0.0 will fail. On other O/S it's necessary to bind the IPv4 and IPv6 sockets separately.

In my evldns package I avoid this by calling setsockopt(s, IPPROTO_IPV6, IPV6_V6ONLY, ...) on sockets that are about to be bound on IPv6. I understand that OpenSSH does the same.