The whole kidney exploding one is amusing, or "The human body was not ment to live off only protien" Or best yet "I couldn't because I'm a diabetic"

What is better though are the excuses I hear alot from LC's who cheat frequently (this isn't aimed at anyone in particular)
"I lost x amount of pounds and wanted to celebrate" (usually upset they put on a couple after)
"I havn't been losing fast enough"
"My freinds/family/whatever told me it wouldn't hurt me"
"it was a holiday" (somewhere in the world at least)
"I NEEDED it"
"I'm not losing weight and upset by it, but I refuse to give up x, y, z"
"But my skinny friend eats it all the time"