BDSM and rape
Coming at it from a different perspective, I see it not as a content danger warning but as a general content warning. I write a lot of fanfiction, and there is nothing more irritating than thinking you are going to be getting a nice introspective piece from the summary but it ends up being yaoi-as-written-by-a-tween: male/male sex in the way only a twelve year old could manage to write male/male sex based on highly specific japanese hurt/comfort dom/sub semi-voyeuristic hanky panky graphic novels. Even if you don't have any personal objection to the content within, a huge problem with internet publishing, especially on sites that don't allow sorting by subject, filtering, or many aids to help readers pick and choose the content they get save perhaps a general search or rating/genre filtering (tumblr dashboard, fanfiction.net, fictionpress, livejournal, etc.) so specialized tags, 'trigger warnings,' character lists and notations, pairing notes, or other summary items are popular. It's considered polite to note anything a person might really, really not want to see before they click after the content break and get a face full of stuff they may not have been browsing for.
But other than that, yeah, nothing makes me more weary than watching people try and take the moral high ground by pre-emptively 'protecting' people without a request. It's better, to me, to offend someone once and apologize and fix what's the matter than to assume someone will object to one's content before even a read. It's always better to me to treat people with understanding, politeness, and respect than slapping on a broad label and calling it a day.
If you're writing fiction, i get it totally. though instead of "trigger warning", as if you knew what someone's triggers are, i'd say "graphic descrption of rape", "tepid sex" etc., :-)
someday you'll have to share if you are comfortable.
My writing is bad, so i don't share. :-)

"Warning: Depictions of Tepid Sex" - excuse me a moment while I retire to the back room to PMSL.
I'm 40. been married for 10 years. they say "write what you know". what do you expect????

TRIGGER WARNING: CONTAINS LINK TO TV TROPES,
"He put his thingie into my you-know-what and we did it for the first time."
I think that cropped up recently when talking about 50 Shades of Bollocks, is it the same one that says something like "Harry Potter totally looked like the lead singer of Good Charlotte"?
I dunno if that's a line in it, simply because every single line in the Legendary Badfic My Immortal is so hideously bad I can't even remember specifics. But it does have gems such as,
"WHAT THE HELL ARE YOU DOING YOU MOTHERFUKERS!" It was............ Dumbledore! ,
Snake and Loopin were in da middle of da empty hall, doin it, and Dobby was watching! ,
“You fucking bustard!” yelled Draco at Vampire. “I want to shit next to her!1” ,
and
Voldemort got a dude-ur-so-retarded look on his face. “I hath telekinesis.” he answered cruelly.
Fuck, man, that has to be an intentional parody. There's no other explanation.
"what are you doing you motherfuckers". She can't say pussy or cunt or even "furry gateway to heaven", but does motherfuckers well.

"I'm not related to Gerard Way but I wish I was because he's a major fucking hottie."
A remarkably good troll is the only coherent explanation.
The remarkable thing about My Immortal is that to this day we're not sure if it's a Poe or not. If it is a Poe, they would have to have written a very, very long fanfiction, made up accounts for not only Tara but her friends, and maintained them... as well as maintaining an entire 'offline life' for Tara; within My Immortal, spats with her friends can be seen. For example, she has a falling out with a friend she wrote into the story and graphically killed the girl on-page, only to forget about it a few chapters later when they'd made up. Not only that, but no one ever stepped forward to claim the story, even when it was at the height of its popularity... something that doesn't really fit with a troll looking for attention.
So while it seems incredibly likely to be a Poe, nobody really knows for sure.
But yeah, it's the Worst Possible Thing that fanfic has, containing all sorts of unmarked things nobody ever would want to see, ever. So writing good summaries, tags, pairing notes, and other stuff is very important in the fic community... to avoid running into stuff like this when you don't want it: not so much the quality, but just the... content.
do you have a link to some common tags? yes, i can google, but asking is so much more social!

Common tags? Well, there's not so much a 'list' of standards. They come in a few categories:
Subgenre tags: People will mark a subgenre of some fics, indicating what people can expect from them. For example, a fic that's categorized as a hurt/comfort fic might be sub-genred 'wumph' (more hurt than comfort) or 'fluff' (more fuzzy feelings and comfort than hurt). Romance fics are further categorized this way, as het (heterosexual romance), slash (homosexual romance, but canon gay couples are almost never referred to as 'slash') or any number of things. Fics that are what-ifs, altered setting, or other elseworlds are labeled 'au' for Alternate Universe. Fics that are close to the canon material without romantic pairings outside of what was originally there are often called 'gen' or 'genfic.'
Content warning tags: These help people avoid fics that may have content they don't want to read and are often one-word warnings inserted after the summary text, such as 'rape,' 'torture,' 'depression,' 'suicide,' 'angst,' etc. Sometimes suffixed with fic, such as 'rapefic.' These warnings are also provided for sexual content, such as 'lemon,' (a graphic sex scene within) or 'lime,' (a homosexual graphic sex scene within, less prevalent)... and in fetish circles can be used to identify specific fetishes.
Pairing tags: In fandoms that have more than one possible romantic pairing, people will proclaim which ones are featured in the material so people can find their preferred set. Usually they are either separated by slashes (Link/Zelda) or smushed together to make one term (ZeLink). Pairings are serious business in fandom so not marking them can make people mad.
Other Fic Elements: Some people mark whether their story has original characters (OCs) or not, too... and other details. For example, if writing a fic about the marvel universe, one would need to list what continuum it's set in.
So an example summary that might appear on fanfiction.net might look a bit like:
The Most Rational Fic, by RationalWikian
KnightOfTL;DR and Godot discuss fanfiction warnings. Genfic.
Most of the crap I read on the internet is marked up like M/f, F/m, F/f, MF/ff...
Anyway, new ambition in life: 1) Become famous and popular author. 2) Troll the fuck out of Fanfiction.net with bad fanfiction of my own stuff.
Nah, not ambitious enough. You should become a neuroscientist and try to top Survey Fail.