Recently, I was cleaning my apartment and I moved a heavy piece of furniture to vacuum the dust underneath it and I found a set of clear plastic retainers for teeth. They must have belonged to the previous tenant who had moved out about 6 months earlier. The retainers looked like they were 3D printed (lots of small ridges on them) and I told my wife that when I had mine (a decade earlier), they were kind of expensive but these ones were 3D printed so they must be a lot cheaper and we can probably just throw them away. The conversation lasted no more than a minute.
Next day, I was browsing yahoo.com and the ad banner had a picture of a clear plastic retainer and was advertising 3D printed retainers. Neither me not my wife had posted anything about the retainers anywhere or discussed it with anyone else or even looked it up on any website (neither of us cares about the subject at all; neither dentistry nor 3D printing). This is the first time I ever saw advertising related to dentistry or 3D printing.
This is not the first time that I've experienced a creepy ad but it was definitely the most disturbing because:
1. It was an extremely specific and niche subject matter which does not relate to me in any way; it was based on an extremely unlikely freak occurrence.
2. I only spoke about it with my wife in an extremely casual way. Neither of us looked it up online on any website afterwards (we verified). The subject matter only occupied our thoughts for an extremely small amount of time.
3. The ad came up the next day after having the conversation.