My favorite "dark pattern" plausibly disguised as a simple bug is that the countdown timer, across multiple platforms I've used YouTube on, doesn't work correctly and very easily will let the ad play for a second or two longer than you intend. It behaves as if it's supposed to count down to 0 one second at a time, but there's a recurring event that is run every second to see if the countdown timer needs to be lowered. However, it's very easy for that event to be ever so slightly delayed or advanced relative to when the timer counts down, and as a result the timer doesn't get decremented correctly. It's very easy to see if you watch the timer carefully; it almost never counts evenly like it should, and it very often skips a beat entirely. (Compare with a simple timer app, which is correctly updating every frame the display renders. This isn't that hard.)
It's very plausibly a bug that a novice dealing with time in a UI would make, and in many other circumstances I'd accept that explanation. But, a bug in such a critical piece of YouTube's functionality, with a root cause I'm pretty sure I've diagnosed just by glancing at it, surviving for years and years across multiple platforms? This isn't a bug. It's policy. At scale, those seconds add up.