When WWE's Kane is on a face run, one thing that smaller (and that'd be most folks) Heels do is to back up, whether retreating from another face or just admiring what they did. And then they bump into an angry guy who's described as 300 pounds of pure psychopath. Cue Oh Crap look.
In fact, The Undertaker's appearance in general is enough to create an "Oh Crap" moment. A favorite among fans is when Undertaker is knocked down, and is lying motionless on the mat. His opponent will start taunting him, before Undertaker will either sit up, or reach up and grab his opponent by the throat. Either way, the opponent's face tells the story.
Kane, as The Undertaker's kayfabe half-brother, is treated as having (to varying degrees) the same "supernatural powers", and thus uses the same style of no-selling. The opponent's reaction tends to be the same.
Notably, The Undertaker is just about the only person who can get this kind of reaction out of Kane, barring a few moments that most count as never happening.
Kane himself got this look after pushing Paul Bearer off a ladder, thinking it was just a mannequin up there.
One moment was during the HHH/Undertaker match at Wrestlemania 27, where after a long grueling bout, HHH tombstones The Undertaker, and he gets his shoulders up at the 2 count. HHH, one of the toughest in ring competitors of the last two decades, literally scrambles back to the ring ropes with a horrified look on his face.
Summerslam 2009. CM Punk defeats Jeff Hardy in a Tables/Ladders/Chairs match for the World Heavyweight Championship title, and takes a moment to stand over his defeated opponent and gloat over his victory...then the infamous gong rings and the arena goes dark. When the lights come back up, Punk comes to the conclusion that nothing happened, smirks, and looks down at Jeff to continue gloating...only to find the Undertaker down there instead. He has a moment of absolute terror before a Chokeslam comes up to greet him.
The audio cue of some wrestlers' entrance music will prompt the "Oh Crap" reaction from an unsuspecting opponent in the ring:
Stone Cold Steve Austin (*glass shatters*)
There's a great example of this at Vengeance (2001) when Chris Jericho who had just eked out a hard-fought (if controversial) victory against The Rock and was being awarded the World title when the (*glass shatters*) and out comes the WWF champ Stone Cold Steve Austin for the Undisputed Championship match. See it here.
Randy Orton demonstrates one of the best examples here. You can see how quickly his face changes from one of celebration to one of OH SHIT OH SHIT GET ME OUT OF HERE.
Likewise, Kane (Not so much an audio cue as a sudden EXPLOSION OF FIRE)
Whenever a Monster Heel was on a bit of a winning streak and bragging that they have beaten just about everyone worth mentioning and they'd like to put out an open challenge to anyone in the back... Cue Chris Benoit's theme.
A world champion whenever a Money In The Bank winner decides to cash it in at their expense (usually right after the champion has just been through a grueling fight with someone else).
One of the best examples is probably The Miz cashing in on a beaten Randy Orton, if only because his facial expression didn't change at all, but his body language pretty much screamed "Oh, you have got to be kidding me."
Averted by CM Punk at Money in the Bank 2011 when Alberto Del Rio attempted to cash in his shot after Punk's title win over John Cena, only to be knocked out by a kick to the head before the opening bell could ring, allowing Punk to escape with his title.
An occasional instance involving two separate Oh Crap moments happens when there is a tag-team match pitting a team of one face and one heel against a team of two heels. The first Oh Crap reaction is from the face wrestler when he realizes that his heel teammate has just walked out mid-match, leaving him at the mercy of his heel opponents. After a couple minutes of the heels beating down the face, the second Oh Crap reaction happens from the heels when the entrance music of another face (often a stronger one) hits, indicating an ensuing Big Damn Heroes entrance by the second face to rescue the beatdown victim.
Another common example is when a heel authority figure is in the ring ranting about something. After being interrupted by a face wrestler, he will threaten to immediately fire the next person that interrupts him. The Oh Crap reaction comes from the heel authority figure when the next person who interrupts him is a face authority figure who outranks him.
A TNA example. Jeff Hardy and Fortune are about to beat down on Mr. Anderson when AJ Styles suddenly stops and does Anderson's signature taunt, Fortune following along before taking out all of Hardy's allies. Jeff Hardy's face turns to dumbstruck shock.
Jeff Jarrett challenged Amazing Red's younger brother to a MMA fight, thinking he'd be smaller than the already small Red. He turns his back to 'give him a free shot'. Red's brother won't have it and taps him on the shoulder. Jarrett's smug face suddenly turns into an Oh Crap when he realizes Red's brother is taller than he is and absolutely RIPPED!
The lead-in to WWE's Royal Rumble. Triple H had been brutalizing and antagonizing Mankind for several weeks, and a few days before, had beaten him bloody. Cue Mankind coming out to inform Triple H that he is not ready to face him in a Street Fight at the event. However...
On the April 11 edition of Raw, Michael Cole gets a rather perfect one after Jerry Lawler defeated Jack Swagger, meaning he has no choice but to face Jerry at Extreme Rules in a match of the King's choosing. This became pretty much his default expression the rest of the night.
And not just that, either. Turns out it's a tag team match, and his partner is Jack Swagger. Normally this would be okay, but this was mere moments after he slapped Jack in the face and blamed him for the loss.
And again when he realized that Jack won't be in his corner vs The King at Over The Limit.
It started a little bit earlier when he realized he'd fallen hook, line, and sinker for Lawler's Batman Gambit, pretty much making Swagger so outright furious with Cole, Swagger says "He's all yours" to Jerry and walks out on Cole.
Pretty much the reaction of the entire Divas locker room to Kharma.
Also Michael Cole's during the Royal Rumble.
On the May 14th edition of Impact!, Immortal has a perfect example of an Oh Crap when they find out that Mick Foley is the Network Executive that's been screwing up their plans for the past few weeks.
Bully Ray had this reaction when Mr. Anderson returned on a Roaring Rampage of Revenge after being put in the hospital by himself and Gunner. It wasn't just that he was back that scared him, though. It was that Anderson, one of the largest hams in pro wrestling, was in Tranquil Fury as he just brutally beat the other guy who put him out into a bloody pulp with a metal chain right in front of Ray and Bully Ray realized he was probably next.
The Undertaker had a very real-life moment of Oh Crap when, during a Hell In A Cell match, Mankind fell through the roof of the cage. The Undertaker thought that he'd seriously injured his fellow wrestler.
An almost-legendary Oh Crap moment came during the 1997 Royal Rumble, as Stone Cold Steve Austin ran roughshod over his competition and was left all alone in the ring. The countdown for the next Rumble entrant finishes, and a familiar guitar screech causes Austin to freeze cold in his tracks.
On Raw 6/18/12, AJ Lee interrupts a match featuring Daniel Bryan and Kane. Not only Kane was surprised, so was Daniel Bryan, who wasn't aware that CM Punk and Sheamus were also surprised just like the announcers, which cost Bryan and Kane the match since Punk sees a chance and takes it.
During the Smackdown Elimination Chamber 2012 match, Daniel Bryan had yet to leave the pod and The Big Show decided the best course of action to get his hands on him was to break into the pod from above! Bryan's facial expression went from smug to pure terror. And there was a minute or so before the pod would open.
Booker T and his match with Rikishi, who's known for his "Stinkface" where, upon downing an opponent in the corner, he would smother their face with his enormous posterior. The look on Booker's face is utterly priceless. As a matter of fact, a lot of superstars got this feeling when they knew what was about to hit them... in the face.
Also, Booker's reaction to that infamous promo. Right after he says the word in the heat of his promo, he realizes what he's done and gives himself a Face Palm.
At Extreme Rules 2012, The Big Show and Cody Rhodes were in a tables match (you can only win by putting your opponent through a table). Big Show was on the apron getting into the ring when Cody dropkicked him, making Show lose his balance and step through the table with one of his legs. Show reacted this way when he realized what had happened, as the ref saw it, called for the bell and gave the victory to Cody.
The main event of the September 23, 2013 Raw with The Shield facing Dolph Ziggler, R-Truth, Kofi Kingston, the Usos, the Prime Time Players, Justin Gabriel, Rob Van Dam, Zack Ryder, and Daniel Bryan in an 11-on-3 elimination handicap match. First, after Roman Reigns singlehandedly eliminates several of the opponents, Ambrose & Rollins are knocked off the apron & promptly have this reaction when Reigns is eliminated in the meantime. Then towards the end of the match, Rollins manages to eliminate R-Truth, and realises he's not only the last man left on his team, but that Bryan, Ziggler & the Usos have positioned themselves on all four sides of the ring, surrounding him.
Raw 11/11/2013, The Wyatt Family interrupted a match with CM Punk and Daniel Bryan dealing with the The Shield, which led to Punk & Bryan playing the two factions off one another so they started brawling whilst they watched with amusement from the outside. Unfortunately, they didn't count on Bray Wyatt figuring out what they were doing and pointing the other 5 men back in the direction of Punk and Bryan; even Jerry Lawler knew what was about to happen.