A cat sits on a mantle in the living room and jumps up to climb onto a window sill above it and it doesn't get completely onto the sill and slips and falls off.
Two cats walks along the mantle above the fireplace in the living room. They go behind a tapestry that hangs from the wall and wrestle and one of them falls off the mantle.
Two girls run into the living room indoors on Christmas morning and go to the mantle to get their stockings. One of them grabs her stocking and knocks something down that causes a chain reaction of every item on the mantel to fall down.