"The Flash" has been a massive success for The CW network, even more so than "Arrow," and the season 1 finale ended on a cliffhanger that had the fans speculating on what's to happen when season 2 comes around. Grant Gustni, who plays Barry Allen a.k.a. the Flash, hinted during an interview with TVLine about the emergence of a multiverse. 

Gustin first told the outlet that Barry Allen's trip to change the past won't go exactly as planned and that's when things start to go haywire. As seen very briefly in the finale, there were quick glimpses of Killer Frost, Hawkgirl, Barry Allen in prison, and the hat of Jay Garrick (Golden Age Flash). 

"What's fun about this show is that there's going to be multiple timelines as we move forward," Gustin explained. "I think we're going to start showing Earth-One and Earth-Two in the near future. There will be kind of different dimensions going on." 

It might be a possibility that the things shown were not from Barry Allen's current timeline but from an alternate universe where things might have gone differently. While Ronnie Raymond became Firestorm in the main timeline, it might be possible that in another dimension Ronnie wasn't affected but Caitlin Snow was, turning her into Killer Frost. 

Carlos Valdes, who plays Cisco, also chimed in about season 2's dip into multiverses and alternate timelines, saying, "It gets a little Rubik's Cube-y in terms of keeping consistency between all these timelines and whatnot but the writers have their stuff together. If I were to trust anyone, it would be them." 

Cisco already showed fans how alternate timelines can affect each other. During the third arc of season 1, Cisco was killed by Harrison Wells/ Eobard Thawne but when Barry changed the past, Cisco was saved. However, he somehow retained memories of what was supposed to happen. This might be explored even more in "The Flash" season 2.