It's probably best to deal with a classic first.
The shocking treatment of Bret Hart at the 1997 Survivor Series is something the Canadian's fan base reviled. Hart obviously thought those disgusted fans would follow him to WCW too. During his post-match tantrum in Montreal, Bret spelled out the promotion's letters as one final act of defiance against Vince McMahon.
Even some of the WWF's locker room believed enough was enough. Mick Foley protested against the screw job and claimed he'd never return. That didn't last, and neither did fan grumbles. Post-Hart, McMahon intensified his newfound "Attitude" and used the controversy to create his own successful heel character.
Seeing Hart struggle to get out of a Shawn Michaels Sharpshooter when he realised referee Earl Hebner had called for the bell is still distressing to Bret's biggest supporters. Treating a man who had literally given his blood, sweat and tears to Vince so poorly turned some people off.