If I have the hero cornered and am about to finish him off and he says "Look out behind you!!" I will not laugh and say "You don't expect me to fall for that old trick, do you?" Instead I will take a step to the side and half turn. That way I can still keep my weapon trained on the hero, I can scan the area behind me, and if anything was heading for me it will now be heading for him.
It also deals with the inversion of this trope (no verbal warning for actual danger):
If I am fighting with the hero atop a moving platform, have disarmed him, and am about to finish him off and he glances behind me and drops flat, I too will drop flat instead of quizzically turning around to find out what he saw.
Some of Wukeywukey's AGK episodes use this, but in this episode, Leopold says, "Oh my goodness! That cactus didn't floss his teeth!" Later on, the dentist says that the cactus Leopold mentioned flosses twice daily. The scene then changes back to the dentist's office, and the cactus is seen flossing his teeth.
Spoofed in Smash Kingdom, where Mr. Game and Watch and Peach are fighting, and the Ultimate Chimera approaches behind her, and Mr. Game and Watch attempts to warn her, but she doesn't listen, apparently misinterpreting his warnings. Finally, he resorts to holding a big sign that says "look behind you!" And Peach then finally understands what he's trying to say, and turns around, only to end up in the jaws of the Ultimate Chimera.
In this episode of Cute Mario Bros, Mario does this to the teacher, and it works so that Mario can change the time on the clock.
In this Super Mario Fables episode, Toad does this to Wario two times.
Happens in the first episode in Sonic Goes to Chaos High School. Sonic says to Amy, "Look! Super Sonic!" Amy falls for it and Sonic escapes.
In Bite Me!, the owner of the pizzeria assumes that this is what the group is trying to do to escape from him. Wrong answer.
The Newgrounds animation Legend Without Zelda BOD1 involved this, in one part, with Navi tricking Mido into thinking that there is a beetle.