I used to be "that guy" that accidentally spoiled things for others. It seriously was on accident, but I've learned to watch what I say now unless people make it clear that they don't mind spoilers.
As for me, for most series it doesn't affect me at all, since I hate "herp twists" in most media anyway, and other spoilers allow me to try to piece together how the spoiler happens before I get to it.
If someone spoiled, say, Halo 4 or the end of Borderlands 2 for me, though, I'd be fucking pissed. I've been careful when looking at Borderlands 2 stuff because I'm still in the early areas. Most things that I'm very interested in and care about spoilers for, I get at launch on midnight, be it movie or otherwise, making spoilers a non-issue much of the time.