You forgot the "I have a cold" excuse. I sing so badly today because I have a cold..... That one drives me nuts as well.
Loved Kevkrom's list! But actually, the "I have a cold" excuse is the only one I have sympathy for. Back when I was doing a lot of singing, I used to have a devil of a time with my allergies. And a bad cold or bad allergies mess up your instrument something awful -- it's like taking a trumpet and pouring a jar of peanut butter into it. Glub, glub! If you have a stuffed-up head cold, the sound simply won't resonate properly in your nasal passages (goodbye head voice and high notes!) and if your throat is messed up, your voice will crack on your "break" notes (the ones in between head and chest voice). And just forget it completely if you have bronchitis.
Of course, all that said, for most people, "I have a cold" really means "I'm scared to death and haven't been practicing like I should!" LOL!
Yeah, a lot of them suffer with colds at one point or another during the process. IIRC Jennifer Hudson suffered through two consecutive weeks of 'having a cold'. I don't recall if she was booted after the second week of her cold. If so, she still sang pretty good and shouldn't have gotten the boot when she did, but I think that season was rigged for Screamtasia from the start.