Recently we had problems with our gateway, connections were dropped and so on.
After a bit of investigation we found that it was due to a bugged game using Javascript which, when it ran on Firefox, opened connections in a loop flooding the connection tracking table in a matter of hours. Once found, it was easy to fix. This was also the occasion to tighten the timeouts values of nf_conntrack a little bit. Indeed 5 days timeouts for established connection doesn’t really make sense when your public IPv4 change every 36hours or so.