Recent arrests of foreigners in Bali for drug offenses highlights the danger of the party scene. According to the Gold Coast Bulletin, 3 foreign men from Italy, Australia and Belgium were arrested at a Kerobokan villa during a raid on a party.
Its quite possible that some of those arrested had nothing to do with drugs, but were simply in the vicinity. In all likelihood the person who sold the drugs, heard about the upcoming party and informed on them. The Scottish fellow I interviewed inside Kerobokan jail a while back claimed he was arrested for being in the vicinity of a drug transaction, but had no part in it.
If you are at a party and stuff is getting offered around you might want to leave.