1. When Barry was interested in the existence of Aquaman.
Sure, this scene wasn't included in the Pilot, but since it doesn't contradict the episode's plot, I'd still consider it to be canon.
2. When Gideon didn’t finish the sentence, but we all knew what she meant.
Two little words, Gideon. Just say 'em.
3. When Wells had a little too much fun making future inside jokes. 
4. When Superman finally showed up after four years of waiting.
5. When Bat-villain appropriation was perfectly acceptable.
To date, Arrow has used the following Batman villains: Clock King, Dollmaker, Ra's al Ghul (and the rest of the League), Deathstroke, Anarky, Deadshot, and the Royal Flush Gang. They're also using Talia Ghul once they return from winter hiatus. (But, in all fairness, the Green Arrow doesn't have too many memorable rogues in the comic books, apart from the Dark Archer).
6. When Oliver Queen was very aware of what Bruce Wayne does in his freetime.
7. When the writers “subtly” hinted at the Green Lantern’s existence. 
The most well-known Green Lantern Corps member, Hal Jordan, was a pilot for Ferris Air who disappeared in an accident, which led to him finding the dying Abin-Sur and being chosen to take his ring.
8. Very subtly.
9. Like, subtle as a flying mallot.
10. When not even the Bat Signal was sacred.
11. When the CCPD decided to commission a Justice League-esque mural.
TELL ME THAT'S NOT AQUAMAN ON THE RIGHT, TELL ME.
12. When future Wayne Tech and Queen Consolidated decided to join forces.
This 2024 newspaper is full of easter eggs. One, in the bottom left-hand corner, hints at the existence of Bruce Wayne.
13. When Kara hinted at the existence of Batman . . . 
14. And then all but confirmed it.
15. When the writers decided “Screw it, we’re using the Justice Society of America.”
The JSA is a canonical comic book precursor to the Justice League, appearing in 1940, whereas the Justice League premiered in 1960.
16. When they were forced to kill of Deadshot, but they brought Floyd Lawton back in “Welcome to Earth-2.”
The DCEU told Arrow that they had to kill off both Floyd Lawton and Amanda Waller because of their plans to use both characters in Suicide Squad. Arrow complied, but The Flash writers decided to use his Earth-2 counterpart. In that universe, instead of a ruthless mercenary, he's a well-meaning police officer with terrible aim. Arrow later brought him back as a hallucination that taunted John Diggle while he was in prison.
17. When Earth-2 Barry had the Justice League on speed-dial. 
18. When someone had a friend from Atlantis.
19. When Barry owned the Hall of Justice.
Since Dr. Wells left STAR Labs to Barry, he also inherited all of its facilities, including a a hanger that bears a striking resemblance to the Hall of Justice, as seen in comics and the cartoon Superfriends.
20. When this valuable rock was all but kryptonite. 
21. When future Batman and Superman were 100% dead.
Rip's from the future, trust him on this.
22. When the name “Oracle” was already taken. 
Barbara Gordon, the first Batgirl, took the name of Oracle after the Joker shot her in the spine and paralyzed her. Under that alias, she became the top hacker and strategist for Batman, Birds of Prey, and other alias. Felicity Smoak, Team Arrow's main hacker, was put in a wheelchair after an attack from HIVE. The DCEU wouldn't let Arrow use the title Oracle in case they used the character in their movies.
23. When Harley Quinn was totally canon. ,
24. When Wonder Woman was the president.
25. When Barry and Oliver were in a LEAGUE of their own.
