I was about 5-10 mins late for dinner with this girl (supposed to meet there at 7:30), and I called her up to let her know, but instead I played it like this (as I pulled into the parking lot) :

Me: "Hey, we said 8:30 right?"
Her: "No! We said 7:30!"
Me: "Shit! Are you already there?"
Her: "Yeah, I put our name down for a table already!"
Me: "Ok, I just got to jump in the shower real quick, and I'll be there in like 25 mins. You don't mind ordering for me do you?
Her: "Are you serious, God!"

As she's continues whining at me, I walk into the restaurant with the cell phone on my ear and a big grin. She spots me, grins back and punches me in the arm a couple times.

I like doing this one even if I'm not running late, like on the way over:
"I'm sorry, tonights not looking so good for me anymore..."
dead silence or WHAT!?
"I think I'm free Thursday tho...."

Sometimes you have to call them back but its pretty funny!