Fake in the Hole

The Mooks are standing back to back, knowing The Hero is near and ready to kick their ass. Suddenly they hear the familiar sound of metal hitting the ground. "GRENADE!", yells one of them and they all drop to the floor. When the explosion fails to occur, they then realize it's just a not-so-dangerous decoy.

Compare Where's the Kaboom?

Examples of Fake in the Hole include:

Fan Works

In the Condor arc of Forward, Mal and Zoe duck for cover only to find that what was thrown at them was an empty clip, with the message of "we could have tossed a grenade".

Film

Seen in the deleted scenes of Serenity, when Mal and Inara escape from the Operative. Mal is throwing a real, but still-pinned, grenade at the Feds to distract them, before gingerly picking it back up as he goes by.
The lead character in Biggles: Adventures in Time is once drawn into a fight after taking a shower - wearing only a towel. He holds off a group of German troops by turning on his cordless razor and throwing it, yelling "GRENADE!"
Tobruk (1967). World War II Anti-Hero Major Donald Craig and his British squad mates used this and Insert Grenade Here to steal a Nazi tank.
In Captain America: The First Avenger, Colonel Phillips throws a grenade at the Super Soldier candidates. While everyone else runs for cover, Steve Rogers throws himself over it. Fortunately, it was a dummy grenade.

Literature

In the Doc Savage novel The Red Skull, Doc deters pursuit by throwing his watch at his pursuers. The crooks, thinking it is one of Doc's gas bombs, break off the chase.
Doc Sidhe: Jean-Pierre does this in the first novel. When the gunmen invade Doc's office, Jean-Pierre throws a paperweight at one, shouting "Stickbomb!". While the thug is trying to get away from the supposed bomb, Jean-Pierre shoots him.

Live-Action TV

In an episode of Hogan's Heroes, the POWs create a distraction by tossing a live grenade into Klink's office but without pulling the trigger cord (the equivalent of the pin on a potato masher grenade). This gives them time to pull off their Zany Scheme while the Germans are diving for cover.
In an episode of Blake's 7, Avon throws a stone into a nest of Federation troops, shouting "Grenade!" The troops reflexively dive for cover, and when they realize it was fake and look up, the heroes have them at gunpoint.

Avon: It must have been a dud. Sorry about that.

A Deep Thoughts segment double subverted this trope, suggesting that if you're even in a war zone, you should shout "GRENADE!" and throw a miniature pumpkin at the enemy. When they see the pumpkin, it'll make them stop and ponder how senseless war is, and while they're pondering, you can throw a real grenade.
Played with in the re-imagined Battlestar Galactica, during the Mutiny arc. Kara and Lee are facing a hallway full of mooks and they only have a flashbang grenade and some guns. So Kara throws the grenade like it's a live one and uses the distraction to shoot the mooks.
MacGyver: In "For Love or Money", Mac removes the explosive core from a grenade and then tosses it at a group of border guards to distract them while he makes a run across the border.

Tabletop RPG

West End Games' Men in Black. During the solitaire adventure in the main rules the alien you're pursuing throws an object at you. You can assume it's a grenade and dive for cover or pick it up and look at it. It turns out to be a package of ~~Ding-Dongs~~ Ding-Dums snacks.

Web Comics

In Blue Yonder Black Dog's men ran when Lena made a cellphone beep and threw it in their direction - soon after they have seen fireworks when she used another one as a Lightning Gun.

Mercenary 1: Did we seriously just retreat from a 15-year old girl with a cell phone?
Mercenary 2: No one has to know.

Western Animation

In the Generator Rex episode "Badlands", Rex throws a can of soda at Gatlocke's gang, causing them to dive for cover as they think it is a cannister of unstable nanites.

Real Life

Truth in Television - In World War II, American troops would sometimes throw rocks to give themselves some breathing room while the Japanese/Germans would dive for cover against a boom that wouldn't come. Until they threw an actual grenade.
An actual Award Order from the Red Army. An unarmed gunner from a burning tank behind the enemy trenches tried to escape back to the friendly forces. He ran into some Germans, but managed to avoid being shot or captured by tricking them with an earthen "grenade". While they take cover, he jumps over the trench and runs. It worked, though he did catch some stray shrapnel and bullets on the way.