Although killing off characters by dropping large objects on them is something normally reserved for cartoons, it still happens occasionally. But the universe (cough, writers, cough) appreciates irony far more than it should, and therefore sees fit that any character who is killed by a falling object will look up and see it just in time to be flattened.
They will usually be alerted by a whistling sound, combined with the rapidly growing shadow of the object that is around them. Of course, they are always comically slow in looking up to see the object... and even once they see it, they refuse to do anything useful such as leap out of the way. They instead choose to stare at it in shock until they are crushed. Of course, it's probably fairly surprising to see a piano bearing down on you in the middle of the sidewalk... but you would think survival instinct counted for something. Sometimes, a character, if trying to invoke the trope on someone they are fighting against, will also point up slightly to hint that they should look up, to which the enemy inevitably falls for it just as they are flattened.
Keep in mind that, as this trope is particularly common in cartoons, where characters are almost impervious to permanent damage, this trope does not always actually result in death.
Sub-Trope of Death by Irony. Super Trope of Falling Chandelier of Doom. Almost always used with Death from Above. Contrast Disney Villain Death.
As a Death Trope, all Spoilers will be unmarked ahead. Beware.
Giro and his henchwolves are all killed in an avalanche at the end of Arashino Yoru Ni.
Films — Animation
When Gru attempts for the last time to thwart Vector's fortress in Despicable Me, he looks up after hearing a bunch of guns cocking, and many missile laser sights cover his face. We then get a non-violent Gory Discretion Shot of a mushroom cloud from far away. As with any kid's movie, the scene ends with Gru after a Disney Death. Sort of.
In Recess: School's Out, after Gus (who acted as the commander of the children faction of the resistance against Benedict's No Recess group) sprayed soda on the ninjas, the ninjas attempt to attack Gus out of revenge, but then Gus tells them to stop, and then points upward. The Ninjas take the hint and look up... only for Spinelli to flatten them with a flying press.
At the end of Ice Age, the evil saber-toothed cat Soto is impaled by several falling icicles.
The Dark Knight has such a scene, though it's a bit different: it was a car bomb. One of the Joker's victims was given an envelope telling her where she was going into hiding, and the only word inside was "up". She looked up... and then the car went up.
Amélie. Amelie's mother meets this fate, courtesy of a suicidal Canadian tourist jumping off a church roof and landing on her.
Transporter 3: The warehouse fight scene ends with Frank Martin hitting a switch that drops a car onto two temporarily incapacitated bad guys, who look up, scream, and hold their hands up (ineffectually).
One of the victims in Final Destination 2 looks up just in time to see a large window literally smash him into a bloody mess.
The next two Final Destination films pay homage to that particular scene by reusing the same camera position showing the victim(s) that are about to be crushed looking up at the camera (and their impending doom). In the third, it's done with a falling cherry picker near the end of the film, and in the fourth, it's done in the opening disaster with a collapsing stone bleacher. The latter of the two notably kills off two people at the same time.
According to urban legend, turkeys and/or chickens are so stupid that if it rains they'll look up to see what's hitting them and drown. Again, however, urban legend. There is a medical condition that effects turkeys, causing them to "stargaze," but it is caused by a nutritional imbalance in their diet and not stupidity. The "drowning in the rain" part comes from farmers joking that if it rains while the turkeys are looking up, then they would drown. And now you know.
Happens to mountain lion Sharptooth in Warrior Cats when Feathertail knocks a stalactite off the ceiling of the cave.
In the first Redwall book, Cluny the Scourge is rooted to the spot in shock, staring up at the bell falling from above.
The show starts off with George looking up at the sky to see a toilet seat from orbit about smite her. Considering that it is moving at terminal velocity and would be more than "fairly" surprising, I do think it is quite justified that she doesn't do more than say, "Oh shit".
Later in the pilot episode, an unnamed woman is crushed by, yes, a falling piano. She does look up, although not soon enough to have gotten out of the way.
Dr. Romano from ER looked up just in time to see a helicopter crash on him, killing him. This after he had survived a helicopter cutting off his arm; they really don't like him.
Supernatural likes playing with this trope. The first time it happens, a piano is dropped on him, but he didn't look up. The second time, both Sam and Dean do look up, but then time freezes and when the air conditioning unit finally does hit the ground, they're not in the same position as before.
Midsomer Murders: In "The Dark Rider", one victim is lured outside his ancestral home and looks up just in time to see a gargoyle toppling on top of him.
Jonathan Creek: In "The Clue of the Savant's Thumb", the killer attempts to topple a stone angel off the roof on to Fariba. She looks up and sees it coming. Her adoptive mother knocks her out of the way and is instead killed by the statue.
From the Lovin' Spoonful song "Pow!":
When they're droppin' a piano from the forty-seventh floor I'm the guy underneath 'em lookin' up...
Parodied in Los Prisioneros's "We are South American Rockers", where some paint-filled eggs were thrown around in the video and the last one, right as the song finishes, is tossed with whistling sound effect and as if it was a piano, as the group members stare in horror.
At the end of Resident Evil: Code: Veronica, Wesker, having just opened a can of whoop-ass on Chris, does a leaping punch to finish him, but Chris uses his last ounce of strength to fling himself out of the way out of Wesker's attack and straight at a lever. Cue loud noises, and Wesker looking up just in time to catch an I-beam with his face right before an entire pile lands on him. Subverted into a Hope Spot, however, considering what said character's already had happen to him.
In the series, there was a returning enemy called a wallmaster, which would appear suddenly in dungeons, scare the crap outta you, grab onto your face, and pull you into oblivion. In most games this would incur some damage, and if you were on the brink of doom, you would crumple to the ground as soon as you reappeared at the beginning of the dungeon.
In The Legend of Zelda: Ocarina of Time and its semi-sequel, The Legend of Zelda: Majora's Mask, a wallmaster attack was foreshadowed by a terror-inducing whooshing noise and a growing shadow under you that followed you around. But whether it be by glitch or purposeful subversion, the best way to instantly avoid this creature was, ironically, by looking up.
The Sims 2 has a very low chance of this happening when sky- or star-gazing: Sims look up shortly before dying from falling satellite. On the plus side, you can sell the satellite in question for fun and profit afterward. No, really.
Asylum: This is a text-with-maze-graphics adventure game for the TRS-80. The plot of the adventure requires the use of this trope. At one point in the game, you find a written note that says "look up". If you do, a piano immediately falls on your head and kills you, implying that there's a concert grand hovering over your head for the entire game just daring you to look at it. To kill an inmate who would otherwise murder you, you have to show the note to him, causing him to look up and get a piano dropped on his head.
The Elder Scrolls: Averted in the Skyrim trailer, in which the fellow looks up and then proceeds to unleash his fury on the offending dragon.
Dawn of War 2 has one moment at the start of a mission where you'll see some Orks standing in the middle of the desert, one of them looks up and remarks, "What's that up in the sky?" moments before your drop pod turns the whole group into paste.
In part two of Nazo Unleashed, Super Sonic rushes into space and then aims back at Nazo back on earth in a manner similar to a meteor. When Super Sonic is close to impact, Super Shadow, who is currently fighting Nazo at this point, points up slightly. Nazo takes the hint and looks up... only for Super Sonic to collide into him and seriously injure him.
Family Guy had something of a parody of this in a Cutaway Gag — the man in "Hiroshima, August 1945" having the Worst Day Ever looks up as a shadow grows over him and a whistling sound is heard, gives a slow, awed "Oh my god..." and gets hit by... an escaped baboon which proceeds to claw his face off. Worst Day Ever, indeed.
A running gag in Metalocalypse is that some unlucky patron (usually a audience member or a production staff) looks up just as chaos erupts. And something sharp flies through the air then impales him in the face.
From Winnie the Pooh: "What do Jagulars do?" "Well, they always call 'Halloo!' And when you look up, they drop on you." "I'm looking d-down, Pooh."
The Road Runner played this trope up, down and sideways with all the various things that fell on Wile E. Coyote's head over the years. He's looked up and then opened a comically tiny umbrella to protect him; looked up, looked to the camera, pointed up and gulped; looked up, seen the rock, stepped just out of the shadow on the ground and been hit anyway... you name it.
The brilliant indie computer-animation short Pigeon: Impossible has the lead pigeon meeting its demise in this manner while eating the all-important bagel for which much of Washington, DC was destroyed.
The 2009 Wonder Woman made-for-DVD animated film features a mook looking up and the last thing he sees is Wonder Woman descending upon him. Given that, in this Darker and Edgier version of the tale, the Amazon kills cold-bloodedly (having done so to a couple of fellow mooks moments earlier and she'll do so again afterwards), it's very likely it was the last thing he ever saw.
Rock climbers never yell "heads up" or "look out" for exactly this reason; people looking up may get hit in the unprotected face instead of on the top of their helmets. Instead, climbers shout "Rock!" (even if what's falling is a piece of equipment or some other non-rock object), which means to get your head down and stand close to the wall. If something very large falls, the shout is instead "Boulder!", which means you should look up, at least so you can see what's about to smash you.
Similarly, at least one construction company has "don'bethere!" as a code word for telling people to vacate where they're standing, without wasting any time to "look out" or figure out where to "move."
Theater. One of the first things they teach on tech crew is don't look up when people yell "heads up," as that's a great way to get a faceful of molten plastic, shattered glass, broken light, falling planks... you get the picture. Run, duck, cover your head, just don't look up.