Often used for a Face Heel Turn; for example, when Bob Backlund made his early '90s return to WWE, he was portrayed as a nice older guy who loved to compete and served as a mentor to the younger wrestlers on the roster — right up until he snapped, putting Bret Hart in the crossface chicken-wing out of jealousy, and becoming an uber-reactionary psychopath. WWE owner Vince McMahon's evolution from goofy good-guy announcer to maniacally lecherous and evilly manipulative corporate boss following the Montreal Screwjob (see Dropped a Bridge on Him) is another good example, also an example of Real Life Writes the Plot.
Though sometimes the face doesn't turn heel, they just reveal that they've Took a Level in Badass. The Undertaker is particularly adept at this. Incidentally, if a heel wrestler got Backlund angry enough, he showed that he could be perfectly willing to throw out the rulebook to teach him a lesson.
John Cena can typically be counted on to be a guy who's pretty calm and relaxed and willing to joke around and playfully mock his opponents, and when competing in fair fights will still find plenty of time to have fun. Piss him off a little too much, though, and he'll start doing things like throwing around 500-pound men, ripping pipes out of the machinery and taking them to someone's face, and trying to run people down with forklifts. He also once dropped 25 CHAIRS on Wade Barrett.
Kofi Kingston is a pretty level-headed guy who is often seen with a smile, but if you push him over the edge, he'll hit back hard. On the 10/26/2009 edition of Raw, after losing the WWE World Heavyweight Championship in an Iron Man match to John Cena the previous night at Bragging Rights, a livid Randy Orton ambushed Kofi, claiming he was the reason he lost, with Kofi clearing Orton's Legacy teammates out of the ring, after getting blamed by them for costing team Raw the 7-on-7 tag team match. A little later, Orton's Legacy teammates, Cody Rhodes and Ted DiBiase Jr., tried to calm him down with a custom NASCAR car. Soon after that, while Legacy was in the ring, Kofi broadcasted himself on the titantron as he defaced Orton's car, his face alternating from mischievous glee to uncharacteristic anger. It got worse in the following weeks.
And exactly what happened in his most recent fights with Ted DiBiase.
Jimmy Jacobs. My God, Jimmy Jacobs. Who would have thought the same person who wore furry boots and played a giant for laughs would turn out to be a complete psychopath who would hang a bleeding Jay Briscoe from the ceiling and shower in his blood?
Melina Perez (fits it more as a Face than when she's heel) walks down the ramp smiling and waving to the fans but when the bell rings she is an aggressive screamer who goes to the limit.
Prior to his 1987 heel turn, Andre The Giant very much fit this trope, with many announcers referring to the big guy as a gentle giant. Whenever a heel crossed his path and used devious means to try to take him out, Andre became brutal and used his aggression to make the bad guys regret their moves. Famous examples came with Killer Khan, who was thoroughly beaten in a legendary stretcher match (the objective being to injure the opponent so severely they need to be taken out on a stretecher) at the Philadelphia Spectrum; and Big John Studd and Ken Patera (after the two knocked Andre out and cut his hair).
As part of the Andre-Studd feud, Andre once appeared on the WWF's Tuesday Night Titans (a takeoff of The Tonight Show) to discuss his feud with Vince McMahon. When McMahon persisted with questioning about Andre putting his career on the line vs. Studd's $15,000 at the first WrestleMania, a clearly annoyed Andre (who did his best to keep his temper and smile) grabbed McMahon by the necktie and told him he would fight the WrestleMania match on his terms before walking off the set.
Andre was also a Real Life example as well. He was a nice guy outside the ring, but some guys decided to try a round of Bullying a Dragon with him. Cue them being chased out of the bar and having their car flipped over with them still in it.
Since his semi-retirement, Mick Foley has generally been a goofball cracking lame jokes. The few people who have truly pushed him, though, are reminded just why he's called the Hardcore Legend.
AJ Lee. Where other divas take home victories with schoolgirl roll-ups she breaks out the Shining Wizards or Sliced Bread #2. From a 100 pound girl, that is pretty unusual. And because Daniel Bryan couldn't appreciate her love for him and continued to demean her and blame her for his loss at WrestleMania, she's now the proverbial crazy chick.
She has no problem putting her love interests through tables (just ask CM Punk and Daniel Bryan) or knocking them off ladders (ask John Cena). If you make AJ mad at you, prepare to suffer.