Piracy

They check the license key hasn't been revoked via a DNS lookup, but not at install time, so often the user installs, uses, then miraculously finds it disabled a few weeks later; then runs to find a new copy/keygen and the whole situation starts again.

(e: they also route the DNS lookup via bonjour if it's running, so you have to keep bonjour segregated from the internet too, which can cause other problems)