ill-error

A phrase anyone can use in order to have a legitimate sounding excuse for making a mistake at work, at home or when you return something, and you can't think of any other reason.

It also helps if your are or have been sick, thus bringing a little cred to your lame excuse. (Can be used in conjunction with it's 2nd cousin: "ill-advised".)

Woman: Wow! What were you thinking? Of course your pineapple is going to spoil if you bought it 3 weeks ago and left it on your kitchen counter. We can't give you a new one.
Man: Yeah, but I wasn't thinking properly...it's an ill-error. I didn't realize it because I've been sick and couldn't come into the store without giving everyone here my flu.
Woman: OK, go pick one out and I'll have someone ring it up at no charge.
Man: Thank You...(with a fake sneeze)