Nah, the worst ads are the ones that advertise the very thing I've recently bought. Almost every time I buy something on-line from Amazon or some company's store, I then keep seeing ads for that very product. Not before I buy it, but after I've already bought.

I've even seen this for what the companies must know is a once-a-decade purchase, like a big lifetime-warranty espresso grinder. It's not like I need a second one! Offer me some accessories instead, dammit.

Hmm, could there be something in the analytics that could be used to claim, in aggregate as opposed to you personally, that the ads influenced your buying decision, hence demanding a higher price, perhaps for future ads? Coz it doesn't make sense why they'd do that.

The weirdest ones for me are ads for lumber. Like, I go to the local lumber yard website to check dimensions of what types of lumber is available when I'm planning a project. That lumber is typically costing $1-$2 per yard, so I frequently only buy lumber for ~$20, and they'll show me ads at least ten times. Their ROI has to be very low?

Adding insult to injury, I only buy lumber from their competitor, who, although having a sucky website where I can't look up dimensions, will let me use their compound mitre saw for free, saving me a $600-$1000 purchase.

That is retargeting and can be extremely effective. Often times it takes multiple touch points to get someone to purchase, so display ad reminders can aid that.