I probably can't tell this story correctly because it is 2nd hand but ...
I was on a browser team. A fellow co-worker decided to add the Fullscreen API to it which means not just add the API but first discuss it in the relevant standards committees.
I'm pretty sure he thought, and so did management, this would be a 2-3 month project at most. IIRC though it was like 18 months, maybe longer.
Some problems that aren't obvious at first
* What is fullscreen mode? Is it a mode for the page, a mode for an individual element? What?
They eventually decided it was for an individual element
* What happens to the CSS above that element when none of its parents are being rendered?
I'm sure that took a while to argue over. Like if it was position: relative or absolute and suddenly it's parent is no longer displayed. What if the parent has CSS transforms? Okay, you say we ignore those and consider it the root element. Okay so does that mean none of the other styles from the parents apply like color or font-family? If some do and some don't we now have to go through every CSS property and decide if it does or does not continue to inherit. I don't actually know the answer to this.
* You have a DOM A->B->C->D->E. C asked to go fullscreen. While there E asks to go fullscreen. User presses ESC or whatever to exit fullscreen. Should it pop back to C or A? Does it matter if E is a video element and they clicked fullscreen? What if C is an iframe does it matter?
* Testing across all devices that support it requires all new testing infrastructure because going fullscreen is effectively something that happens outside the page not inside so testing that it actually happened, that a user can actually exit it correctly, requires entirely new test systems that were not there for previous APIs. Then multiply by 5 at least (Windows, MacOS, Linux, Android, ChromeOS, ...)
And so even though I'm sure everyone ended up understanding that, it turned out to be way more work than anyone expected. Yet, in the back of their minds it was arguably always "this is taking way longer than it should, goals not met" or at least that's how it seemed to be perceived.