Out of interest, If I purposely try and find bugs/vulnerabilities in the site, is that permitted/encouraged/discouraged? I haven't found any, but occasionally I run out of things to do so a live system would be interesting to poke at. Thanks.
If you mean Stack Exchange, then no - it would be illegal in many countries, and you could expect all manner of problems if you tried it.
From this earlier query by @AviD on the topic, we got the official SE response:
Sorry, we've discussed this internally and it's not something we (Stack Exchange) can sanction. In addition to the legal ramifications, we're not comfortable with encouraging attacks on systems with people's private information.