Poirot took a quick step forwards.
“You should not do that. Mademoiselle–“
The young woman started nervously at the sound of his voice. The revolver slipped through her fingers. She was standing by the edge of the pool and the revolver fell with a splash into the water.

Her mouth opened and she uttered an “Oh” of consternation, turning her head to look at Poirot apologetically.
“What a fool I am,” she said. “I’m sorry.”

The Hollow by Agatha Christie