Just read a book in which a character had, in the back story, been saved by a ghost, of a man who had just died. We had a touching moment where the rest of the squad admitted they knew it and had felt rather hurt that he had kept it secret, and someone threatened to use it against him. But -- no ghosts in story. What's the point of having a guy see a ghost if you don't do something with it? Plenty of other men died in the books, and there were plenty of things he could have been told. And there were plenty of other things he could have been threatened with, and the moments with the men could have centered about, that didn't bring up such expectations.
I still remember a book where I introduced a character in the opening chapter to bring news back. Except that he was too developed for that. So I killed him in the second chapter. Well, he stayed dead, but he didn't stay away. I had to keep alluding to him.
Then there is stuff that doesn't scream "Significant!" When the reader runs across it again in the final chapter, it is a wonderful surprise to see how it all ties in. And sometimes it leaves a vague dissatisfaction when the reader doesn't know what wasn't developed that should have been -- but knows that sometime was missing.
Of course, what really helps with pulling the stuff together is to start with a plot that draws the reader in even while you are throwing oddments at him for his amusement. If you set your character wandering about a wild, wacky, wonderful world for the vaguest of reasons, it's probably best to keep to the travelogue and throw more bright sparkly stuff at the reader rather than work out a plot with all the stuff in the first half, and introduce it only in second half.