If you mean just physically gross and not morally repugnant, it would be this. One summer when I was over at my then-sweetie’s apartment (which he shared with a few other guys), we realized we’d all been noticing a kind of stench for several weeks. And the fruit flies were pretty bad, too. In fact, it was worth investigating, so we did. Turned out that the Stench Epicenter was underneath the kitchen sink, located in a black plastic garbage bag. He actually opened it to see what was inside. The smell was overwhelming, and hordes of fruit flies flew out. Inside was – well, it might have been potatoes (or onions?) at one point, but we couldn’t tell. It was alive with maggots and fungus and it was vomitous. He ran outside to throw it in the dumpster. College male apartment + forgotten food + summer + anaerobic decomposition is not an equation I ever want to deal with again.