The only spoiler that ever really annoyed me was over 30 years ago, when the Alec Guinness version of "Tinker, Tailor..." was on TV. I'd missed the first run, but I thought I'd either catch when repeated (no TV recording in those days), or - more likely - read the book.
Then bloody Terry Wogan on R2 came out with: "Well, that last episode was good. Who'd have thought that ****** was the 'mole'?"
I subsequently didn't read the book or watch the DVD until a few months ago, as a result of that - and I still remembered who it was!
I guess he thought it was OK, as it had just ended, but still. I normally wouldn't care, but this was first-and-foremost a "whodunnit". Knowing the denouement didn't make it not worth watching, but it did take some of the edge off it.
Since then, I can't say I've ever been bothered by TV spoilers, but that's probably because I don''t watch much serial drama.