We've got a thread talking about ridiculous crossovers already, so let's talk about ridiculous ways to facilitate these crossovers.

Of the ones I can think of off the top of my head there is the ever popular "Character(s) receives a special invitation to attend Hogwarts school of witchcraft and wizardry, despite 1) not being 11 years old, 2) coming from a country other than England, 3) using a completely different magic system than wizards from the Harry Potter world that is most likely incompatible and/or 4) Not actually having magic at all prior to the story." All of these except points 1 and arguably 2 can be applied to most subjects of "Character becomes this year's Defense Against the Dark Arts teacher" too, except add that the character is usually not part of the wizarding world before Dumbledore asked them to teach at his school.

Another one that I just thought of recently was the lesser known X-men device of having Cerebro mistake the characters from the other canon for mutants, thus causing the X-men to go try to recruit them. This only makes sense if the characters in question fit the marvel mutant definition of having been born with strange abilities in a way that can be explained as being caused by the x gene and without those abilities being due to being non-human or half-human. There's a Yu Yu Hakusho fic that comes to mind where this device is deliberately invoked to allow Yusuke, Kurama and Hiei go to America under cover to investigate a demon who was going around pretending to be a mutant himself. It involved Koenma removing a ward that protected demons on earth from being detected by those means or something like that. The problem with this though, is that Yusuke's powers come from being part demon and Kurama and Hiei are both a demon in a human body and a full blooded demon respectively.

Finally there is the less canon specific method of making certain characters previously unknown relations of other characters in the other canon. The likely hood of this working well depends on the canons and the characters used, and if the author doesn't take those things into consideration it can get pretty ridiculous.

What other crossover plot devices can you guys think of?

The one I encounter a lot is the device of using no device. No explanation required--Dragonball Z and Harry Potter have always taken place in the same world, dontchaknow! There are some canons that could believably coexist (it wasn't much of a stretch for me to cross over G.I. Joe and Alien vs. Predator, since they're both modern day and the AVP business is supposed to be very very secret), but sometimes you just have Sailor Moon and the Doom Guy inexplicably running into each other at the Handi-Mart and nobody batting an eye.

The other one where there is no explanation are the 'bamf' fics. For no apparent reason, people suddenly appear in another universe, where they automatically speak the language, etc.

Buffy - Slayers are actually... I have yet to see this done in a manner that could plausibly explain things like how a new slayer is only ever called upon the death of the old one.

I thought Willow broke that system in the last season so that all the potentials can become Slayers whenever? Or are you talking about stories that take place before then?

When I was 12-13 I wanted to write a Neopets/Stargate SG-1 crossover. I can't recall if I actually did it, but I was new to fanfic writing and it seemed like a fantastic idea at the time. After all, I reasoned, there wasn't anything to indicate that there wouldn't be a stargate somewhere in Neopia.

Read an X-men and Friday night Lights crossover once that had Jason Street suddenly developing mutant powers--at seventeen. Correct me if I'm wrong, but isn't that shit supposed to manifest at puberty, say, twelve or thirteen? It was otherwise well-written, though.

Oh, another one you see a lot is 'Mad scientist character is experimenting with cross-dimensional technology; said tech goes horribly wrong/goes horribly right, and drops characters from other media into mad scientist's world'.

I once saw a Torchwood/True Blood crossover, which existed solely because (1) Both shows took place in the real world, just in different countries, and (2) The author wanted to turn Jack into a vampire. While Torchwood has traveled outside of Wales to investigate unusual happenings (though only in the BBC radio dramas), they've never gone to the United States, nor could I imagine there being any reason they would. I don't know much about True Blood, but my son watches it, and apparently, the vampires keep pretty much to themselves and try to avoid attracting attention. They certainly wouldn't be anything Torchwood would be interested in (unless they're alien vampires).

And you couldn't turn Jack into a vampire anyway (at least not permanently) All someone would have to do would be kill him, and he'd automatically revert to 'default' status.

Apparently Buffy the Vampire Slayer crosses over with anything, including Looney Tunes, Star Trek (TOS), and Power Rangers. To say nothing of the usual suspects like Twilight.

Uh, Eileen, I've done one of those. I thought it was two, but the Trek crossover was with Enterprise.

Well, I didn't say it was bad, necessarily, just . . . weird. Where are they? I'll read them.

I've seen at least two Kiki's Delivery Service fics in which it's mentioned that as soon as Kiki finishes her year on her own, she's going to go to Hogwarts. In one it didn't even figure in the plot: the story (never finished) started when Kiki was seventeen, and it was casually mentioned in passing that Kiki had spent a year studying at Hogwarts. There was absolutely no reason for mentioning this, as it never connected to the plot in any way.

In the other, Kiki and her parents spend a year at Hogwarts: Kiki studying with the first year students although she's at least two years older than they are, her mother as (IIRC) Potions mistress, her father as Professor of Muggle Studies. The author was at some pains to try and reconcile the magical systems that really are completely incompatible (in the Potterverse, if your broom is broken and you can't mend it you buy another; in Kiki's world, you have to make another), and the author didn't realize that after her year on her own Kiki would have to be at least fourteen years old, so having her show up eleven-year-old Draco made her look more than slightly like a bully.

Really though, I do prefer the idiotic plot devices over the ones where the characters just randomly show up with no explaination at all.

There are some fandoms that can successfully mix. All the rest can only be mixed for lulz.

Time-Travel. Don't ever forget inexplicable time-travel.

Or cryogenic freezing.

But RB, that wouldn't be inexplicable. Time travel is central to the show's premise. There's a reason why it's the O blood of crossovers.

I'm surprised no one's mentioned the plot device of "Canon of Fandom A walks unknowingly into the setting of Fandom B". Unless that falls under the "I can't believe it's not a plot device" concept.