Crossovers are a strange area of the TV world, clashing together worlds that most viewers had considered completely separate. Usually just fleeting cameos, they can often be a very welcome meta in-joke. And occasionally they can be so bizarre that you question everything you've ever believed.
Here are eleven crossovers ranging from the inspired to the absolutely insane.
The Simpsons and Duckman
Homer Simpson appeared briefly at the end of Haunted Society Plumbers episode of the cult cartoon show Duckman (skip to 20:45 to see Homer). Klasky Csupo, who produced Duckman, had animated the first two seasons of The Simpsons, although the producers still had to seek permission from Simpsons creator Matt Groening and the Fox network. Surprisingly, they agreed to the cameo.
Seinfeld and Mad About You
Mad About You was never really big in the UK, but it was huge in the US. It was also not averse to crossovers with other sitcoms on NBC. Probably the best was this episode, The Apartment, in which Paul (Paul Reiser) is persuaded to offload the lease on his old apartment to the current tenant, who turns out to be Seinfeld's Kramer (Michael Richards).
Mad About You and Friends
And sometimes Mad About You characters would go to other shows. Ursula, Phoebe's twin sister on Friends, started out as a supporting character in Mad About You, but when Lisa Kudrow got the part of Phoebe on Friends, Ursula switched shows and became a small part of the Friends universe. In the Friends episode The One With Two Parts Helen Hunt and Leila Kenzie from Mad About You made a brief appearance on Friends, in which they confused Phoebe for Ursula.
Bonus fact: In the Friends episode The One With The Blackout, the blackout was caused by Helen Hunt's character Jamie in an episode of Mad About You that aired immediately before.
Friends and Caroline In The City
Friends and the Lea Thompson sitcom Caroline In The City did a swap-over of cameos. Matthew Perry appeared in a scene in a video store in an episode of Caroline in the City, while Lea Thompson appeared in the Friends episode in which Chandler and Joey leave Ross's child on a bus. Neither character is actually named in their appearances.
Buffy and Angel
Only barely qualifies as a crossover, since Angel is a spin-off of Buffy The Vampire Slayer, so the characters started out in the same series. Nevertheless, the reunion of Buffy and her one true vampire love was huge for fans. Buffy went to visit Angel in LA and in the process of killing a demon, Angel somehow became human again. The pair enjoyed a night of bliss, but the next day Angel discovered the consequences of being human and had to go back in time to undo the change. Yes, that sounds ridiculous, but it was so sad, you don't even know.
Teenage Mutant Ninja Turtles and The Power Rangers
Don't even know why or how or what's going on with the female turtle. Let's just accept that this happened and move on with our lives.
Super Mario Bros and Inspector Gadget
The Super Mario Bros Super Show was a horrible spin-off of the computer game series, with terrible jokes, terrible acting and moderately terrible animated sequences. In this bizarre crossover, a live-action Inspector Gadget popped in for an operation. Performed by a pair of plumbers. The production levels are exceptional. Just look at the moment Inspector Gadget loses his hammer hand, which he is clearly holding in his actual hand. No expense spared.
The Mask and Ace Ventura
Both The Mask and Ace Ventura: Pet Detective were given cartoon spin-offs after the success of the movies. Neither cartoon character was voiced by Jim Carrey, meaning a cross-over was much simpler than it might sound.
The X Files and The Simpsons
The Simpsons has had hundreds of celebrity cameos, but the famous guests are usually playing themselves or characters written especially for the episode. In The Springfield Files, David Duchovny and Gillian Anderson voice animated versions of Fox Mulder and Dana Scully as they investigate supernatural goings on in Springfield, and cope with the even weirder things that happen in Homer's head.
Rugrats and Aaahh!!! Real Monsters
The Rugrats was a huge hit for animation house Klasky Csupo. Aaahh!!! Real Monsters, which was a very weird and more adult cartoon, was more of a cult favourite that could never quite settle on which age group it was intended for. Suggesting that the makers viewed it as a kids' show, the two series crashed together in this Halloween Rugrats episode that probably upset a few children.
Green Hornet and Batman
Showing that crossovers are nothing new, The Green Hornet and Kato appeared in two episodes of the 1960s Batman series. So Bruce Lee, who played Kato, once fought Robin. And lost.
(Images: YouTube, AllStar)