Both "The Flash" and "Arrow" did many official crossovers together, but nothing prepared fans for a teaser that was shared on Twitter about a crossover scene. Many reports say that the scene will be shown at next week's episode of "Arrow."

Executive Producer Marc Guggenheim posted a photo on Twitter which revealed the crossover scene, Cinema Blend reported. The photo shows The Flash himself running past Captain Singh - and it looks like he is either placing a placing a post-it note on his computer screen or removing it from the screen.

The teaser apparently does not explain why The Flash would make a cameo in "Arrow," but fans speculate that Oliver might reach out to Barry for help to track her down. Others say that Barry and his team could be helping Team Arrow win over Prometheus. It is also possible that the Flash might actually need Green Arrow's help with in tracking down a villainous metahuman. Barry has asked Felicity's help in the past, and some say it's likely that he would need her help again.

Entertainment Weekly said that the crossover is not the only surprise that "Arrow" has in store. Guggenheim also teased that an actor is going to reprise his role from one of the other shows in the season finale of "Legends." He also added that a character from one of the other shows on "Legends" will be seen in a way that fans have never seen before - and may never see again.

Guggenheim also shared about a possible Black Siren return, and hinted at the new potential Black Canary's abilities. He drops even more spoilers when he discussed whether "Legends'" move to Tuesdays behind "The Flash" has played a role in future storylines. "We're going to be seeing the Black Flash on Legends," he confirmed.