SPOILERS ahead for The Flash season 4 and Arrow season 6
The season 4 premiere of The Flash seemingly reveals two Arrow characters who survived the explosion on Lian Yu in the latter show's season 5 finale. Ever since Barry Allen (Grant Gustin) was introduced to The CW's DC TV universe and spun off into his own television series, the network's Arrowverse lineup has often used crossovers and references to its other series to establish a shared world. While this includes major events like the annual four-show crossover between Arrow, The Flash, Supergirl, and Legends of Tomorrow, it also includes cameos or characters name dropping each other.
When each of the Arrowverse shows wrapped up their respective seasons earlier this year, Arrow ended on arguably the biggest cliffhanger out of the four series - and in the show's own five-year history. Oliver Queen (Stephen Amell) traveled to Lian Yu to confront Prometheus, who had kidnapped all of Oliver's loved ones and held them captive on the island. As a failsafe, Prometheus rigged the island to blow up if Oliver succeeded in killing him. So, while Oliver and his son were safe on a boat when Prometheus killed himself, all of Oliver's friends and family were still on the island - and it was unclear who lived and who died. Surprisingly, The Flash season 4 premiere revealed at least two of the survivors of the explosion.
In the season premiere of The Flash, 'The Flash Reborn', Cisco Ramon (Carlos Valdes) is in the process of trying to save Barry from the Speed Force prison - which he entered willingly at the end of season 3 in order to stabilize the Speed Force. When explaining the technology he created to save Barry, Cisco reveals the fellow scientific minds of the Arrowverse he consulted during his development process. He said, "I've consulted with Harry and Tracy and Felicity and Curtis."
Felicity and Curtis are, of course, referring to Arrow's Felicity Smoak (Emily Bett Rickards) and Curtis Holt (Echo Kellum), who have seemingly survived the explosion on Lian Yu. Since 'The Flash Reborn' picks up a few months after the previous season finale, and presuming the season 3 finale of The Flash and season 5 finale of Arrow took place roughly around the same time, that means Felicity and Curtis are at least two of the survivors.
Of course, there is the possibility that the timelines of The Flash and Arrow are somewhat off and Cisco consulted with Felicity and Curtis prior to their trip to Lian Yu - but that would overcomplicate the DC TV universe and would need to be addressed before the two shows unite later this year for the next Arrowverse crossover, Crisis on Earth-X. So the simplest conclusion is that Felicity and Curtis survived Lian Yu and the Arrow spoiler is an unfortunate consequence of so many moving pieces in the Arrowverse. (In all likelihood, the dialogue was written into The Flash script as a fun nod to the larger Arrowverse and the fact that it's a spoiler for the Arrow premiere went unnoticed.)
That said, Cisco's mention of Felicity and Curtis isn't the first indication that the two Arrow characters would survive to see the season 6 premiere. In fact, the poster for the Arrowverse crossover, Crisis on Earth-X, includes Curtis' Mister Terrific, while set photos from the mega-crossover show a different Felicity spoiler. So, those fans paying attention to Arrowverse news likely knew Felicity and Curtis survived Lian Yu, but for viewers who don't follow the news as closely, The Flash premiere's Arrow spoiler likely came as a surprise. Still, fans will have to tune into the Arrow season 6 premiere to learn the fates of everyone on Lian Yu.
The Flash season 4 airs Tuesdays @8pm on The CW. Arrow season 6 premieres Thursday @9pm on The CW.