Not Safe For Anyone

Seriously, if someone has marked something NSFA, that does not mean they are giving you 'fair warning' it means that they were unable to remove the material from the world, and therefore have no choice but to leave a warning behind them. This is not a secret invitation, challenge, or dare to actually look at the thing. It is not safe for anyone, anywhere, anytime.

Remember, what has been seen cannot be un-seen. If you value yourself (which you should) then you will take heed of any ominous NSFA markers and head immediately and un-curiously in the other direction. Whatever that happens to be.

Examples of NSFA material:

Basically most of 4chan's /b/ board.