Do you mean "weird" as in "sick and wrong" (ie, should never have been written/disturbing content), "weird" as in "creepy/unusual", or do you mean "weird" as in "strange pairings"?
In terms of weird pairings...I've seen too many of them out there to list. I don't even bat an eye at some of the "out-there" pairings fanfic writers come up with.
If you mean "weird" as in "Twilight Zone-like creepy", my favorite is a Gundam Wing story (3x4/QuatrexTrowa pairing) called "Gestalt" (I'm not putting a link to it because it has a warning of some adult content despite the fact that it's rated "T" and not "M". I typically skip over any citrus scenes in stories, so I don't recall what it might have to cause that warning).
If you mean "weird" as in "hilariously OOC and just overall strange", there's a Naruto story called "Naruto Primer". It's lighthearted and hilarious, with no real pairings, but still "weird" in one sense.
If you mean "weird" as in stuff which is a little over the line and that I avoid like the plague but still see when I scan descriptions, that'd be:
-Mpreg
-Genderbender (only exception is with Ranma fanfiction; The type I'm describing is: "Character A wakes up and is suddenly the opposite gender", or: "Character C has been hiding their true gender"...typically used to stay away from lesbian/gay pairings. Genderbenders were extremely popular before gay/lesbian pairings became more widely accepted)
Many incest/twincest fics also make me shudder (I saw them frequently in the form of YohxHao fics in the Shaman King section).
"Weird" to the point of crossing the line...again, I don't actively seek out these stories, so I usually wind up pulling back in horror due to the fact that the author failed to warn the readers (or just a glance at the description), and that would be RoyxEdward (FMA) pairings, or anything similar with an adult/child age difference (like SnapexHarry in the Harry Potter section).
I tend to think of myself as pretty open minded when it comes to fanfiction, but there are still some lines which should not be approached, let alone crossed.
