Films — Animated
Dory: "Duck!"
Marlin: "That's not a duck; it's a PELICAN! AAHH!"
— Finding Nemo (the reason he's scared is they're both fish)
Films — Live-Action
Literature
Gregory "Greg" Heffley: "Duck! Duck!"
Frank Heffley: "Where? Where?"
Western Animation
Buster: Brain, can you help me come up with a new villain that the Bionic Bunny could fight?
Brain: Hmmm, let's see. Squids have been done. Turtles have been done. How 'bout a— DUCK!
Buster: Nope. Episode number 2436, the Mean Mallard Master.
Brain: No, Buster, duck!
— Western Animation/Arthur