Done a lot by the Final Girl in the first Friday the 13th (1980) movie. Granted, the third time she made a point of checking long enough to see a puddle of blood next to the killer's head, and promptly took that as adequate evidence to assume it was safe to leave the body and go. Uh, not so much.
Averted by Tommy Jarvis in Friday the 13th: The Final Chapter, where after confusing Jason by dressing up as the latter as a child, then successfully driving a machete through Jason's head, sees the killer's fingers twitch, and stabs him another dozen good times. While shouting "DIE! DIE! DIE!" Unfortunately, this marked him becoming the next Jason.
Part VI: Jason Lives featured the town sheriff trying to avert this; repeatedly hitting a downed Jason, who just shrugs it off. Why nobody except Jarvis did this in the first film, nobody knows.
Part VII: The New Blood went to ridiculous extents with this. Psychic girl shocks Jason with power line, he walks it off. Girl drops porch on Jason, Jason's back in the 40 seconds she took to lower her guard. Girl hangs Jason with power cord and throws him through floor, he grabs her when she looks down. Girl sets Jason/house on fire, house explodes, Jason comes out on fire, but still fine. It takes resurrecting her dead father to fight Jason to finally put down the masked bastard. Every single time, she acts like, "Oh, the guy that just walked off getting a building dropped on him is definitely dead this time."
Subversions are extremely rare, but here's a few.
Jeepers Creepers. The characters are in their car, trying to escape the Creeper. He leaps in front of them, so the girl driving floors the gas pedal and runs him over! She then stops, puts the car in reverse and backs over him. She then does it again going forward, then back and forward before speeding off. She smooshed him up real good, but it sadly wasn't enough. At least she tried.
Averted in the movie Death Proof where they shoot the antagonist and then after he attempts to get away they actively hunt him down.
Averted many times in Evil Dead 2. Ash swings off the head of his girlfriend with a spade and then buries her. When she comes back he retaliates, with a chainsaw. Ash made a point of teaching everyone the lesson in time for Army of Darkness:
Similarly, in the modern B-MovieMonster Man, the film ends with the two survivors stealing the monster truck driven by the titular horrifically deformed Satan-worshipping redneck, ploughing it into him, and then spending about five minutes of screen time (and 8 hours of actual time) running back and forth over him with it, reducing him to a great smear of mashed flesh and crushed bones. Which, because his Satanist sister used Black Magic on him to make him into her useful servant, still has an intact mouth in it burbling"You can't kill me... you can't kill me...!"
Scream (1996) is guilty of this too, as noted below, it does finally get its act together in the finales of the first two movies. In the first, they hold a gun on the killer's "dead" body in case he's about to come back... he does try to, and promptly gets drilled right between the eyes. In Scream 2, they don't even wait for the killer to try to come back before shooting the corpse in the head, just to make sure.
Halloween (1978) did it TWICE. Same girl, same killer, leaves him "dead" with his knife right next to him, and there can't have been more than five minutes between them.
The reverse also happens in just about every movie in the series; they kill off Michael in some increasingly ridiculous and stunningly final way, only to have him survive. At the end of the second one, he was actually intended to have died, but when the Michael-less third movie flopped, they had him come back in number four. And five. And six. For the record:
Halloween (1978): After everything else he's subjected to that night, he's shot six times at damn near point blank range and falls off of a second story balcony only to run away so quickly they don't see it happen.
Halloween II (1981): Shot repeatedly, then blown up when an entire room full of ether ignites in a hospital. Which happens about two hours after the end of the first movie, so add them both together. Remember, this one technically did kill him.
Halloween 4: The Return of Michael Myers: After another night of minor injuries, he is shot roughly twenty-six times, I counted, with what looks like some kind of assault rifle, then dropped down a mine shaft, along with a lit stick of dynamite, which explodes.
Halloween: The Curse of Michael Myers: At the end, Michael is stabbed with about a dozen syringes containing something that looks like antifreeze and it beaten over the head with a lead pipe until green slime is oozing out of every hole in his face. When the camera cuts back, he has vanished completely, despite apparently having no eyes or brain left. If it weren't for the Continuity Reboot he'd still be alive today.
The villain isn't really Michael Myers - it's pure evil in his form. Kill it all you like, it'll wake up eventually. There's a reason Michael's actor is always credited as playing "The Shape".
In Red Eye, the protagonist continually tries to run away after a single blow. She succeeded in slowing him down temporarily, but that's all.
Justified; the first time she attacks him and runs, the time limit and situation made it impractical to stay and finish him off, and in the instances after that, she has already called the police and is just trying to survive until help arrives. Also, she is a civilian fighting an experienced terrorist, so if she lets it become a full-on fight she is almost certain to lose.
Avoided in Sin City, with the gruesome fate of that Yellow Bastard. Kevin gets a similar treatment, having his limbs and head sawed off and his guts fed to a wolf.
Being a send-up of slasher flicks, it happens multiple times in Scream (1996). For example, the killer has a victim cornered in the garage. She manages to nail him with beer bottles and whomp him with a freezer door and he goes down. She then tries to squeeze through the dog door, with predictable results. Watching it just makes you want to yell, "Finish him off, you idiot!"
Averted, then played straight in The Hills Have Eyes 2. They "kill," the mutant, make sure, then he gets straight back up. This happens two or three times in a row, seriously that guy was absolutely Made of Iron.
And in the remake of the first one. The protagonist has just smashed the last mutant's head in, and blasted him repeatedly with a shotgun...and he still gets back up.
Subverted in Spider-Man during the wrestling match. Bonesaw gets a chair and repeatedly whacks Spidey over the head, pausing only to let him try to struggle to his feet, only to smack him down again. However, his ultimate mistake is stopping to go and get another weapon (a crowbar), giving Peter enough time to get his wits together and start counter attacking. Of course, having the proportionate strength and endurance of a spider helps, Bonesaw likely didn't figure that Pete would be able to take all that punishment and still be able to fight back.
The Empire Strikes Back has Luke retrieve his lightsaber, chop off one of the Wampa beast's arms, and then run out into a Hoth blizzard. He could have just finished the beast, checked the cave for more, and then stayed there until the storm passed. It may have been an ice cave, but it had to be better than just running out randomly. This may not seem like much of an example until, during the EU, Luke goes back to Hoth and nearly gets killed by the same, dismembered Wampa beast.
Justified, albeit possibly by Retcon: The EU has also elaborated a bit on the nature of the use-of-force guidelines (no, not that Force) that Jedi are meant to abide by, and killing the Wampa after wounding it badly enough that it's no longer a threat would have been a violation. (Jedi get some extra leeway for non-sentient lifeforms, but not a lot.) Which of course leads us straight into another trope...
The last two deaths in Diary of the Dead are a direct result of this trope. Both the camera man and one of the woman knock down their friend-turned-zombie when he's chasing the latter, hit him once on the head, and leave him there, despite the woman in particular having been shown as perfectly able to finish off a friend-turned-zombie, her boyfriend no less. Naturally, the zombie gets back up, and eventually knocks one of the characters into the bathtub with a hair-dryer, and takes a bite out of the camera man.
Defied in Zombieland, where the Genre Savvy Columbus makes a rule about "Double Tapping", IE shooting the zombies more than once and making sure they're dead.
Columbus: In those moments where you're not quite sure if the undead are really dead, dead, don't get all stingy with your bullets. I mean, one more clean shot to the head, and this lady could have avoided becoming a human Happy Meal. Woulda... coulda... shoulda.
Mentioned in The Quick and the Dead. Up until that point Russell Crowe's character has been able to win all his quick draw matchs with only one bullet (he's a priest and refuses to kill them) until his next match makes a Badass Boast about one bullet wouldn't put him down; he's right it takes two.
Slumber Party Massacre III runs on this, as one by one the victims hit the killer once and try to run away. Fail, rinse, repeat. This continues even after they manage to blind him.
Averted in Iron Man 2, when Tony Stark is attacked by Ivan Vanko on a racetrack. Happy Hogan and Pepper drive to the rescue and drive right into Vanko, smashing him up against a wall. Happy then proceeds to back up and ram him into the wall several more times. Of course Vanko is tricked out with high tech gadgetry which keeps him safe, but the trope is nonetheless Defied with Tony yelling "HIT HIM AGAIN!"
In Live Free or Die Hard, McClane's problem with Mai could have been solved much more quickly had he bothered to confirm she was at least out of it for good.
Shooter. Subverted in the love interest pumping a few more bullets into the downed mook who had threatened to cut her throat.
In the first Star Wars EU Boba Fett novel, both Boba and Jango pull a Aerial Canyon Chase (involving missiles) on pursuing starfighters in canyons, and the narrator makes sure to point out that someone tried the same trick on Jango once but it only worked the first time.
It's bad enough that the victims do it all the time, but in one episode of Law and Order: Special Victims Unit, a detective does this to a man who's trying to rape her. It not only was a blatant use of this trope, but it also destroyed that detective's toughness cred in viewer's eyes.
Averted and inverted in the pilot for Firefly, where Dawson, the Fed, takes out Book with a single blow, then hits him a few more times, just to be sureout of vindictiveness at the Shepherd's earlier assault on him (just look at his expression).
Subverted at the end of "Shindig."
Heroes Claire at the beginning of Season 3 when Sylar tracks her down.
And then her father, who slits his throat when the powers are off and then just walks off. And then Claire again in the finale — she shoves some glass into the back of his neck, and then leaves, even though she personally brought Peter back to life after the exact same injury.
Also, Mohinder in season 1, who knocks him out by slamming a giant map into him. Awesome. But then he just leaves him there, despite having shot him, point-blank, while he was helpless last time he had the chance. The fact that this didn't work really should have reinforced the necessity of doing it right this time.
Subverted in "Buffy vs. Dracula". After Buffy stakes Dracula and everyone appears to leave the castle, Dracula regenerates from a mist - only to have Buffy immediately stake him again, saying, "You think I don't watch your movies? You always come back." He tries to regenerate again, but disappears after she says, "I'm standing right here."
Although according to the continuation comics he's still around, so presumably he just waited until she left and got up again. Of course, he's also an occasional ally now, so twice was apparently enough to make him think better attacking her.
Averted, though unintentionally and in a rather creepy way in Babylon 5. During his escape from the Babyverse version of "Minilove" where he was tortured and mentally abused to near BSOD, Sheridan kills a guard. He then continues to blankly pound PPG blasts into the dead body until his friends stop him and lead him away.
LOST, multiple incidents. The characters know that humans are much more durable on the island. As such, they fail to make sure their enemies aren't going to come back.
Little House on the Prairie: The 1981 episode "Sylvia," where the main character, a 15-year-old physically mature girl is raped by a masked assailant; she is impregnated as a result. Later, Sylvia's father is able to shoot the rapist ... but he only wounds him, and the rapist is able to get away, his fate left unknown.
During the pre-mid 1990s era, when wrestling operated as strict kayfabe and most wrestling programs consisted of several matches with established wrestlers taking on jobbers, promoters might turn an already imposing heel wrestler and turn him into an unstoppable monster by having him continue to physically punish a jobber long after scoring the victory. Often, this consisted of the heel using his finishing hold — always powerful, and used solely to inflict great injury — one or more times after the final bell. (Usually, the Heel would then take on the top babyface, who — after taking (sometimes) the finishing move several times — would fight back and defeat the heel.)
Done in Mass Effect 1 with Krogans. When fighting Krogans, most of them, when "dead", will drop to the ground, then regenerate some health and get back up. Definitely hurts in the midst of a multi-foe firefight.
In the story, the Krogan follower Wrex discovers that the main Big Bad/Dragon Saren has developed a cure for the Turian genophage which renders 90% of all Krogan women infertile. He very seriously considers whether Shepard is doing the right thing in trying to blow up the facility. If you choose the "Renegade" option, you can argue with Wrex for a bit, then end up in a Mexican Standoff, in which you can choose to simply kill him rather than convince him otherwise. You bash him with your gun to knock him down, then shoot him... several times.
After Saren dies, Shepard's first instinct (after taking control of the Citadel so that the fleet can destroy Sovereign) is to tell his/her companions to "make sure he's dead." One of them shoots Saren before confirming that he is. Of course, that doesn't stop Sovereign from animating the Reaper tech that was implanted in Saren.
In the Silent Hill games, if you don't either stomp on the enemies, shoot them, or whack them again after they've been knocked to the ground, they'll get up and attack you again.
Eternal Darkness has the aversion as an important game mechanic. Once you knock an enemy down, you can perform a special, extra-violent attack to make really sure they stay down. Since the enemies are Eldritch Abominations who wreck your mind just by existing, the decisive finishing move makes you feel better and boosts your Sanity Meter.
In most of the Resident Evil games, zombies will eventually die for good if they take enough damage. However, in the GameCube remake of the first game, while the zombies will eventually "die" after taking enough damage, if their heads aren't destroyed (or, failing that, if their corpses aren't set alight), then their corpses don't simply disappear once the player leaves the room. Instead, they remain, and, after enough time has passed, they will come back to (un)life... having mutated into the far deadlier Crimson Heads.
There is also the title character of Resident Evil 3: Nemesis. The player can knock him down after pouring buckets of lead into him, but he will always get back up and running within short order. He can even be knocked down in the middle of a fight, only to get back up again just as a player goes to check if he dropped any special items, which usually earns them a walloping.
In Batman: Arkham Asylum and its sequel Arkham City, you have to use a special takedown move to make sure a mook does not get back up (and most of the time, they will).
Quake's zombies can be downed by enough conventional damage, but it's a temporary respite at best - a few seconds is all it takes for them to come back up. Unless, of course, you follow up with a rocket and explode the corpse into giblets, or just blast them into bits the first time around.
In Hotline Miami, when you knock down an enemy with a door or thrown weapon, you need to confirm the kill by ground-and-pound or the enemy will just get back up. This doesn't apply to guns.