Sure. If you dont have any kind of screen on the instake a small fish could find itself stuck to the powerhead intake. This will/can cause internal injuries.
__________________ Remember dont tap the glass, your fish will think you're an idiot-Anonymous mother
My fish will swim in front of them and get blown away, but it doesn't injure them. My orange starfish got sucked up by a PH and it was shooting out parts of his body, I got him off it and he lived for about a week before he died.