As someone who has written fan fiction which involves rape and sexual violence, at the time (2003) I was unsure how to label it. As I have friends who are abuse survivors, I went with "Warnings for sexual violence." in the header and "This story contains mature themes, including sexual violence. Proceed with caution." in the author's note. In this case, because while it did not contain graphic depictions of rape, the story does include rape and is I knew (from doing months of research on PTSD and surviving sexual assault, and also frankly one of my betas opting out due to the subject matter) it would be triggery for survivors. And I want people to be able to opt out regardless of their reasons if the subject matter was uncomfortable, distasteful, or upsetting in any way for them. That said, I had implied dubious consent in "That Old Yeh Shen Story" which it never occured to me to warn for at the time, and the recent discussions have convinced me that I ought to. Because it's the sensible thing to do. And I don't have any problem with that.
If there is anything I've written which you as a reader had felt completely blindsided by, please never hesitate to let me know. Not to put the burden on readers, but I would prefer to know and go back and label appropriately (for future readers), than continue on blindly in the dark.
I can't undo any harm I've done, but I can damn sure try and avoid doing harm in the future.
ETA: I have now seen this from a few folks, and it seems sensible. I don't refer to Content Labelling as "Warnings", because of the value judgement implied by the word (especially where 'slash' and 'het' are concerned).
Content Labelling System:
Spoilers: I generally include in the synopsis which episode of a series a story is set during or after, and in the case of cast changes on something like Spooks, for example, I'll reference an episode/season number. But I generally assume, if you'e reading fan fiction in a fandom, you're watching/have seen the series. Also, I make the same assuption about films, that you wouldn't be reading if you didn't already know all the plot spoilers in the source.
Character deaths: Nope.
Slash or Het: I don't always label stories with a pairing unless it's a romance. I define "romance" as "is your A-plot about a romantic relationship?" rather than whether or not the story contains references to canon relationships, non-canon relationships, flirting, or even a romantic B-plot, because I tend to focus more on the A-plots. I realise this marks me out as both a) ancient in online fandom terms and b) out of step with the rest of online fandom. But I haven't really got around to revising my position yet when it comes to my own archive I maintain. So you're stuck with me only labelling romances with flavour-of-content labels on my own sites, tho on Livejournal communities where headers require pairing codes regardless of what kind of story it is, I'll include them.
Violence and traumatic non-sexual events: It depends on the source. For example, I wouldn't necessarily post a warning on a Doctor Who story about Lucy Saxon/The Master story unless it contained graphic depictions of violence or rough sex, since pretty much both consensual rough sex and an abusive relationshop are implied and or explicit in the source. Also, aside from some body horror and implied horror, I rarely if ever include graphic depictions of violence. But were I to, I'd probably label on a case-by-case basis, and in relation to the source.
Rape/Sexual Assault/Dubious Consent: Yes.
Underage sex: Probably only if it's a minor paired with someone over the age of consent. Which (aside from Buffy/Angel implicit in the source) I don't think I've written? But if I did, then I probably would. But not necessarily if it's consensual between sexually mature teens still under the age of consent (which I need to check up on, cos I always assume it's age 16). I'd have to ponder this one. It too might be on a case-by-case basis.
Incest: I've never written graphic incest, consensual or otherwise, and I most likely never would. That said, I do have one Firefly story which contains one-sided incest subtext which I probably ought to label.