You might get an angry commenter if it's just a minor thing. Or if it is very terribly obscene (sexual situation, gore, violence, explicit language) someone from Tapastic would probably message you and tag the page NSFW. I don't imagine the consequences would be too harsh for first time offenders accidentally not tagging a page. There's not really a section on this topic in the FAQ, but you could always write them an email if you want the definitive answer.
Where as if you tag it and there's nothing offensive at all happening, you might get more people skipping past the page, or maybe a random person asking why it was tagged.
Either way, use the tag thoughtfully. If it's not needed, don't use it, but if you feel the page might offend some of your readers than use it. Though, an upside don't cross necklace probably doesn't fall under NSFW, like what @DavidDaneman said. 
