The Montreal Screwjob.

In Montreal Canada on November 9, 1997, a match between Shawn Michaels and the soon-to-be-leaving Bret Hart was set to take place during WWE's pay-per-view Survivor Series.

Leading up to the match, Harts departure to rival company WCW was made known to WWF management. Being the WWF World Champion meant that Hart would have to lost the title to rival Michaels during an upcomming event. When the idea of losing the belt at the next upcomming pay-per-view, Survivor Series, was set to take place in Brets home country of Canada. Hart, not wanting to lose on his home turf, disagreed with the idea. It is said they they came to an agreement in which Bret would retain the title at the pay-per-view, but then lose it at the following televised RAW.

Vince, who has been burned in the past by his superstars, feared that after the pay-per-view Bret would not hold his own part of the bargin and leave the fed with the title.
So Vince called in Michaels, HHH, and a close group of advisors four days before the pay-per-view and planed the screwjob. The plan was that Hart would be put into a submission by Micheals and the ref would then call for the bell as if he had submitted.
The ref for the match, Earl Hebner, was not informed of the new ending until shortly before the match begun.
During the match both men and the ref were knocked to the ground. Micheals was first to rise and placed Bret in his own finisher "The Sharp Shooter". At this point the match director was heard shouting directions for the ref to get up. After doing so, Hebner called for the bell which was followed my Mcmahon shouting "Ring the mwuah bell!" The bell rang, Shawn's music played, and he was announced as the winner and new champion.

The ref quickly left the building, stopped by the hotel, and took a plane home, while Hart, who was understandably angry, spit in the face of Vince. Micheals was instructed by Vince to "pick up the mwuah' belt and get the mwuah out of here." Fans threw garbage into the ring as McMahon and company left, leaving heart to sign "W C W" and "I love you" to the crowd as he made his way backstage.

Backstage, many wrestlers were shocked and upset over what had happened. The Undertaker confronted Vince and demanded that he personally apologize to Bret. While doing so, Hart was angered by Vince's explanation and warned Vince that if he did not leave, he would get hit. After this, both men struggled and Hart got the upper hand by punching Mcmahon in the face. After which Vince's wrist was broken during the comotion due to someone accidently stepping on it.
Bret's wife at the time, Julie, confronted HHH and others who she believed were in on it as well.
Shawn Michaelsand HHH were confronted and assulted by angry fans outside the arena and in the lobbey of their hotel.

While many thought this would have been a nail in the coffin for WWF and a great opporunity for WCW, it only propeled McMahon to become the top heel in the company naturally and lead to one of the greatest storylines: Steve Austin vs Vince McMahon, and WCW failed to do anything to truly capitalize on the situation.

Bret Hart was considered by many, even himself, to be extremely underused in WCW for 2 years and toward the end of his big push, he was retired after an injury in the WCW ring with Bill Goldberg.