A recent episode of Arrow season 7 seems to have spoiled an upcoming episode of The Flash season 5. As both shows exist in the Arrowverse, they're always making references to each other. Usually those references are easily understood by viewers, but it seems in the latest Arrow episode, a character made a reference to an event in Central City that hasn't happened yet.
Early in the Arrowverse's history, Arrow and The Flash characters would periodically crossover for an episode here and there, but those crossovers have become more sporadic due to the complexity of each show's story. But at least through cameos, television news reports, and phone calls, The Flash and Arrow have occasionally found ways to acknowledge the latest developments in Central City and Star City - and it looks like they still plan on continuing that going forward, because it just happened again.
While searching for clues to the identity of the Star City Slayer in Arrow season 7 episode 13, Dinah Drake decided to call The Flash's Captain Singh of the CCPD. During the conversation, Singh name-dropped a Batman villain while trying to provide Dinah with some encouragement. Singh wasn't actually able to give Dinah any useful information but did mention an incident involving the CCPD that put several police officers in the hospital. According to Singh, the super-powered villain responsible for the attack was subsequently apprehended by Team Flash.
The reference to the attack raised eyebrows, simply because it doesn't match anything that's occurred in recent episodes of The Flash season 5. This, of course, suggests that the incident in question will take place sometime in the near future. It seems strange that the timelines for the two shows would be off, considering the air dates for both seasons. The season and midseason premieres of The Flash aired one week ahead of Arrow, but for the most part the two shows have been in sync with each other in terms of scheduling. However, The Flash did not air on November 6, due to the U.S. election. So, some details could easily fall through the cracks.
Singh's explanation for what happened at CCPD doesn't give too much away about the threat Team Flash is about to face. Singh only describes the villain as a "new metahuman freak." This eliminates the next two opponents Barry will face: King Shark and Gorilla Grodd. Neither are human, and both are established foes of Team Flash. Since The Flash doesn't return for a few more weeks, little is known about the episodes that follow "King Shark vs. Gorilla Grodd", so it's hard to predict which metahuman will be attacking CCPD. In any case, thanks to Captain Singh, Team Flash's victory over their mystery new adversary has been confirmed weeks before they even make their debut.