bait and switch
When an image appears to be one thing, but is actually another. Or, when the outcome of a situation is the opposite of what you expect. Often, this is a situation that appears to be sexual, but is actually totally innocent.
If it's a pool, tag this at the page of switch only.
See also
- good end
- come under the blanket (meme)
- expectation vs. reality
- Danbooru pool #1229 (Misleading Thumbnails)
- Danbooru pool #5019 (Unexpectedly Good)