I had one where the ad looked totally 'normal' for a Craigslist ad. It went something like "15yo horse for sale. Pasture. Lots of other horses come look. All must go ASAPP!!!!!!111!!!!!!one!!!!! Make offer."
I kid you not; the ad really read like that.
Anyway, I saw the picture with it and I was like, "hmm... something's not right about where the horse is standing". It was in a very unkempt field, the fence was broken, the horse was severely underweight and I saw the hooves were in really bad shape (not trimmed whatsoever; the poor beast was standing pigeon-toed, so bad was its condition). I sent an anonymous message to a horse rescue requesting that they simply take a look to ensure the horses were doing okay... and I never heard back.
But two weeks later, on my local news, 60+ horses were seized from a private ranch for severe neglect (malnourishment, abuse, lack of medical attention, etc etc). Coincidence? I think not. Regardless of whether that was because of what I did, I'm glad I acted on my gut instinct.