additives. There are reports of 12 people already dead this week in Bali from drinking arak with added methanol(methylated spirits) and other chemicals. It has been added to locally made arak to produce a quicker cheaper effect, unfortunately fatal. It is generally only sold in village warungs so normal tourist drinking spots should be unaffected.
see Jakarta Post article
http://www.thejakartapost.com/news/2009/05/28/alcohol-poisoning-claims-six-more-lives-two-critical.html