The most common case of this would be a shared seedbox. This is not recommended at all as it can result in lost accounts but we do not disable accounts for it as long as it does not happen on home connections.
Servers
You are not allowed to use the same IP to access the site on multiple accounts, but you are allowed to seed/leech from the same IP on multiple accounts. Basically, accessing the site in your browser from a shared IP is not acceptable, but using a shared IP to download and seed torrents is OK.