It may come from crosspost bots or entered manually.
Some cleanup made for :
- fisting
- reach-around
- foreskin
- scat & feces (sometimes it's poop)
- bestiality
- rape
- child_on_child
- tentacle_rape
- frottage
- tribadism
- fingering
- large_insertion
- tentacle_sex
- threesome
- necrophilia
- happy_sex
- double_vaginal
- navel_sex
- prolapse
- multiple_paizuri
and several others

Danbooru wiki is a good place to discover a meaning, e.g.
danbooru.donmai.us/wiki_pages/reach-around?z=2
is not a usual hug.

In the ideal world post with such tags has to be
(auto?)checked and corrected right after arrival.

Some other tags USUALLY have heavy-NSFW connotation
but CAN happen here in casual meaning
- spitroast
- mating (by cats etc) & testicles (animal)
- uterus
- penis_in_panties & penis_in_swimsuit & covered_penis (for traps)
- suspended_congress
- strappado
- 69 & 96
- vagina_dentata (horror characters)
- oral & oral_invitation & cunnilingus_gesture
- handjob_gesture & fellatio_gesture
- penis_measuring & penis_envy
- penetration_gesture (not cleaned yet)
- gaping (not cleaned yet)
- facial (not cleaned yet)
- imminent_sex & foreplay
- orgasm (?)
- imminent_rape & torture (?)
and plenty of guro, bdsm / sex toys & peeing tags.

Maybe we can do nothing with it but manual cleanup.