A good one a friend of my brother's came up with during high school. They were playing foosball during lunch hour and he called someone an a-wordhole. Only he didn't actually call them an a-wordhole, he used the actual a-word and put the word "hole" after it. But a teacher overheard, and started walking towards him to reprimand him, so to cover his tracks, he said, "Uh, yeah. So he put the gas in the gas-hole." Best. Cover-up. Ever.