I've rarely been fond of reading stories written in collaboration by authors who took turns alternating sections. It often seems obvious to me where the seams are--though I've never actually sat down and checked with the authors, so I could be totally wrong about that, seeing seams where there aren't any. But even knowing that there are two or more authors makes me look for seams, because so many collaborative stories seem to have them.
One of the things that feels like this kind of seam to me is when something new comes up in the story, and we're told that it's been there all along but there's been no evidence of it.
For example, after a difficult knife fight in which the protagonist barely escapes with her life, in the next scene she gets attacked again. She narrates: "Then I remembered my trusty death ray, which I carry with me at all times. I pulled it out and killed him with it."
Why didn't she use it in the knife fight? All sorts of reasons could be grafted on, but the real reason is that the death ray didn't exist until another author decided to add it to the story.
And my impression is that often, a lot of the fun for the authors in writing collaborative stories is exactly this kind of thing--each author, in turn, throws in new stuff for the other author(s) to deal with. It's a fun game.
But the result too often reads, to me, more like the authors playing a game with each other than like a coherent story. That can be a source of reader pleasure for plenty of readers, but it's not my cup of tea.
The collaborative stories I like best tend to be, I suspect (but again I don't know), the ones in which one author takes the whole story (after the first draft is done) and does a cleanup pass on it--polishing out inconsistencies, removing bits that have become extraneous, trying to match prose styles, etc. Sometimes more than one author does that, in turn, and that's fine too.
(There are also, of course, lots of other ways to collaborate that don't have this particular kind of problem. I'm certainly not saying collaboration is inherently bad; we've even published one collaborative story, and I've liked plenty of others too.)
Anyway, my focus here was not meant to be on collaboration per se, but on the adding-new-things-later issue. This comes up in serial formats all the time, such as TV. For example, there was an episode of BSG in which an old lover of Apollo's shows up; if she had been introduced several episodes earlier, and we'd known all along that she existed and was an important part of his life, it could have been a really compelling episode, but because we had never heard of her before, I found it difficult to suspend disbelief about the idea that she'd been an important part of his life all along. (And iIrc, she was never mentioned again.)
It's tougher to fix this kind of thing in TV (or comics, or a series of novels, or a roleplaying campaign) than in a single work; you can't go back and add stuff to earlier episodes. And if you come up with some really cool new thing that you wish had been part of the story all along, I don't want to say you shouldn't add it. But there are better and worse ways to add such things.
My preference is to do so much worldbuilding up front that you can drop hints all along; then they pay off really well down the road. A couple months ago, I re-read all of Girl Genius so far, and was pleased to find that stuff that had been cryptic in the first few issues turned out to make total sense years later. I find that really impressive.
But it's not always feasible. So another approach is to drop hints that you, the author, don't know the significance of yet, and figure it out later. That can be exceedingly hard to do well; most of the time, it results in dangling plot threads and disappointing non-payoffs. But when it's done well, it's great. The Alias writers were brilliant at this--my understanding is that, at least in the first two or three seasons, they would throw in plot hooks without any clear idea of what they meant, and then six or ten or fifteen episodes later, they would come up with great resolutions to those hooks. If the writers for a given series can pull this off, it makes them look amazingly organized and thorough in their worldbuilding.