Well, if someone makes a thread about the Destiny trilogy and specifically mentions that they haven't read it (just to pick an example), I imagine most folks would try hard not to spoil the books for that person. That having been said, accidents do happen, especially if the thread goes more than a page or so - it's unfortunate but not always avoidable. This especially applies to novels such as The Eternal Tide
which are almost impossible to discuss without spoiling the novel for new readers.
It's always a good idea to bear in mind that, no matter how familiar you may be with something, there's always going to be someone out there who isn't familiar with it. I managed to keep my kids spoiler-free on the subject of Darth Vader's true identity (and Luke's parentage) until I felt they were old enough to sit through the saga a few years ago. They knew there was a character named Darth Vader but they had NO idea who he really was until watching Episode III. That was fun