If you repeatedly exploited either the game or a bug in a wide-spread mod to screw with servers, the consequences could be as severe as a key deactivation.
If you took advantage of an uncommon, crap-onned, or private mod on the server to screw with it, the result would be a controversial drama topic, split between people calling you a richard for doing it and people blaming the host for running the add-on. These opinions would probably not be tied into each individual's personal beliefs so much as their opinions of you and the victim, so you may want to take that into account. You probably would not get in trouble; if I were to take a guess at Badspot's stance, I assume he'd blame the host.
If you used third-party software to screw with another person's computer by any means, it would probably have the same effect as above, but with a more interesting drama topic and probably a higher chance of being banned depending on how you pulled it off.
