I think the stupidest excuse my boss heard once was that I was going to be late because the cat peed in the laundry basket.

We used to have a dress code where I work, and I'd just done all my laundry the night before, all my work clothes, pants, etc. Everything was folded in the laundry basket, and one of our cats decided it was a rockin place to piddle. I tried to ask if I could come in casual, the answer was no, so I had to wait until laundry finished before I could come in to work.

He still pokes fun at me about it as the lamest excuse ever.