hentai waiting to happen
1. A statement said when a creature with tentacles or tentacle-like appendages appears in a non-hentai environment. When saying this, the speaker is referencing the tentacle rape hentai genre, and suggesting that the creature could be the rapist in such a hentai. Often funny when directed toward innocent or unsuspecting creatures that usually would never be associated with hentai. 

However, some people have been known get offended when tentacled creatures are judged by their appearance.
(example 1) atheist: Have you heard of the Flying Spaghetti Monster? If you want intelligent design "theory" to be taught in schools, we should teach flying spaghetti monsterism as well!

creationist: Oh please, "touched by His noodly appendage"? Sounds like hentai waiting to happen.

athiest:... How dare you judge the flying spaghetti monster by his appearance!

(example 2) Two guys are playing a fighting videogame.
Guy 1: *chooses octopus-man as his fighter*
Guy 2: Hentai waiting to happen.
Guy 1: WTF DON'T JUDGE OCTOPUS-MAN BY HIS NAME AND/OR APPEARANCE!