Funniest excuse was when one of my brothers and I caught the other brother dressing in drag when we are in all in Junior High / High School.

My middle brother and I lived in the basement of our house, and the youngest brother lived upstairs. We were watching TV when the youngest brother comes down stairs and tries sneaking in to the laundry room. Ok weird, but whatever. 10 minutes go by and we dont see him come out, so we decide to go see what he is up to. We walk in and there he is trying to dress in our little sister's clothing... He got freaked out, and of course the other brother and i started cracking up and making fun of him.

He ended up saying his excuse was "I saw the ghost of ____ (or recently deceased dog), and I felt indecent so I had to throw on some clothes! ____ (sister) clothes were the only ones lying around!"

Of course being in high school we made fun of him pretty hard for it, to the point where my dad got really pissed off and told my other brother and I to stop.

I haven't brought it up in over a decade, because I don't want to be a douche, but that was the funniest / best excuse I've ever heard.