Crossovers between shows airing on different networks or taking place in incompatible universes are harder to pull off.
Now and then some can cut through the red tape and pull off a mindblowing crossover between shows we never thought could crossover.
More then previous Arrowverse crossovers, Crisis On Infinite Earths had a lot to prove. It had to top the previous crossovers. It had to be as epic as the comic it took inspiration from.
Most of all, since Arrow was ending, it had to be a fitting swan song for Oliver Queen.
Great as it was seeing characters from all Arrowverse shows act together, those in charge of making Crisis on Infinite Earths rightly recognized this story needed to be bigger than just the Arrowverse.
They got characters from shows long over -- Smallville, Birds of Prey, and Adam West's Batman -- to make appearances.
Movie versions of Superman and The Flash showed up, which was surprising because tv verses are usually kept very separate from movie verses.
Best of all, Diggle and Mia accompanied Constantine on a journey to visit the Devil himself -- Lucifer, as played by Tom Ellis.
Rumors of it leaked before The Flash Season 6 Episode 9, aired, but it didn't take away what it represented.
Crisis on Infinite Earths was epic enough to cross network lines. Also, even if we didn't know it, we needed to witness Lucifer mispronouncing Constantine's name.
Will the Arrowverse and Lucifer ever cross paths again? It's unknown, but knowing it could happen adds another layer of excitement and possibility to those shows.
Lucifer isn't the only character who has made the perilous journey across network lines.
Community Season 2 Episode 9 mentioned Abed was watching Cougar Town.
Abed delivered a monologue about being an extra on Cougar Town on Community Season 2 Episode 19. It served as an unexpected vehicle for the exploration of Abed and Jeff's characters and as a payoff to the joke of Abed liking Cougar Town.
Only it wasn't the end of the connection between Community and Cougar Town.
On Community Season 2 Episode 24, Cougar Town actors Busy Philipps and Dan Byrd were spotted in the background of a crowd scene.
Then things really became meta on Cougar Town Season 2 Episode 21 when Danny Pudi, Abed's actor, could be seen sitting near Laurie and Travis until he suddenly makes a run for it.
Obviously (and delightfully), Pudi was playing Abed as an extra on Cougar Town.
These were only cameo appearances, but the crossover was amazing for a few reasons. It happened between two shows on rival networks. It showed the admiration both shows had for the other.
Plus, it was a prime example of how Community and Cougar Town go the extra mile to deliver payoffs to recurring jokes -- making you appreciate both shows all the more.
Community and Cougar Town are very much distinct, unique shows, but they complement each other well.
Something I wouldn't think to apply to Bones and Sleepy Hollow until the two shows crossed over on Bones Season 11 Episode 5 and Sleepy Hollow Season 3 Episode 5.
Bones takes its share of artistic liberties, but it's a drama centered around using science to solve crimes. Introducing the supernatural would break the rules embedded into the show.
Meanwhile, the supernatural is crucial to Sleepy Hollow. It is, after all, a show about Ichabod Crane waking up in the 21st century to stave off the apocalypse.
How do Bones and Sleepy Hollow crossover without ruining the way their universes work?
When the Bones characters were involved, the crossover did a good job teasing the supernatural without actually revealing the supernatural (and without making Brennan and Booth look stupid for not putting the pieces together).
As fun as it was too see Brennan, Booth, Ichabod, and Abby figure a way out of an Indiana Jones and the Temple of Doom-like trap, what makes the crossover special are the character moments.
Brennan and Ichabod debated magic versus science and the pros and cons of mixing work with pleasure.
Booth and Abby bonded over their shared feelings about their jobs and their respective relationships with Sheriff Corbin.
Compelling character dynamics can make even the most unlikely of crossovers work.
The only thing wilder than a crossover between a supernatural show and a non-supernatural show is a crossover between a live-action show and an animated show.
Supernatural proved it was up for the challenge when it crossed over with Scooby-Doo on Supernatural Season 13 Episode 16.
Supernatural balanced both the absurdity of Sam and Dean getting pulled into a Scooby-Doo episode and how freaking awesome it was for the out-of-the-blue (even for them) thing to happen.
It's an intriguing episode because it challenges the characters from both shows. Dean, Sam, and Cas need to adapt to being animated.
The Scooby gang gets confronted with heavier subject matter -- like their mortality -- than they're used to dealing with while solving mysteries.
It was also a hilarious and affectionate Scooby-Doo tribute.
The risk of doing a crossover with Scooby-Doo paid off for Supernatural.
What could have been a disaster made for a great TV episode and demonstrated how a live-action show could go animated without losing its identity.
Over to you, TV Fanatics!
Which TV crossovers caught you by surprise?
What shows do you think would make for an unexpectedly good crossover?
Hit the comments below.
You can watch The Flash online at TV Fanatic.
Becca Newton is a staff writer for TV Fanatic.