Ad blindness has fucked me so many times on web UIs.
There have been a couple of particularly vivid incidents where the company put some sort of interaction on a page and positioned it and shaped it like an ad. So I bitched about how that button wasn't on that page and it was literally front and center (specifically, slightly right of center with text wrapped around it), but positioned like an ad so I didn't see it.
I've experienced this with gigantic download buttons. Took me several seconds of scanning the page to realise the huge green download button was the real thing and not a scam ad as they usually are.
There have been a couple of particularly vivid incidents where the company put some sort of interaction on a page and positioned it and shaped it like an ad. So I bitched about how that button wasn't on that page and it was literally front and center (specifically, slightly right of center with text wrapped around it), but positioned like an ad so I didn't see it.