In the episode of I Love Lucy where the Ricardo and Mertz family realize their entire life is in a 'rut', so they split up and Lucy and Ethel stay together for a week, and Fred and Ricky stay together. About two-thirds of the way through the episode, Lucy and Ethel end up trapped outside on the roof of the building... This is where this takes place.

Lucy sees Mrs. Sanders light on way down below, so to get her attention she tries through rocks at the window.

On the first throw, if you watch it in slow motion or even in regular motion, you will notice that as she throws the rock 'all the way down' towards the lit apartment, you can see it bounce back up and go to the right of the screen. It's pretty noticeable even with out slow motion (if you pay attention).

Also, as she throws it downward, you hear it hit something right away, and then it bounces off and you can hear it hit the ground again.

The next throw hits a window right next to the ladies house, even though she throws it practically in the same place as the other one.

You know that this is supposedly 5 flights up because Lucy tells Ethel to pretend it is only a foot up rather than 5 flights up when crossing the board.