simply imagine if the scene you write can be too heavy for some type of people.
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violence - some people have a big problem with it - you anyway cant be too explicit but in case you feel you are on the border its good to offer to skip
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scenes of a traumatizing situation like the attempt of rape or abusive parents atc - a reader who went through such experience himself might be reminded of his trauma by this so such scene are good to offer to be skipped
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people bullied especially bases on some specific - like being mistreated because they are gay or disabled - again some people might face such things in their life and might feel triggered by such a scene
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self harm , suicide - this is sensitive too
and well I think in post above its practicl all listed. 
On the other hand, you anyway have to be “in” the episode guidelines to make the story +13 so none of above mentioned you cant anyway write in a hardcore version. But if you make the extra effort and add the skip option some readers might appreciate it.