Well, maybe not pure alcohol, but let's say someone who had a heavy night of drinking and didn't drink water but only flammable beverages... Is there any chance his or her urine could be flammable?
The liver breaks down about 90% of alcohol consumed, with only about 5% excreted in the urine. To become combustible, the ethanol concentration in urine would have to approach 50% at room temperature, though 20% would combust at around body temperature. However, conversion of ethanol into acetate by the liver generates one molecule of water for each molecule of ethanol consumed. Since 18 times more alcohol is metabolized by the liver than is excreted in the urine, the water from all that liver metabolism would render the urine too dilute to combust even if it were not supplied by the diet.