Sometimes they have convolutions up their sleeves. A character makes a comment, and you realize that that's one more thing that the characters will have to face.
It's a complication. Subplots and minor characters thicken the world and the story. Another convolution just makes it longer. A sequence of events, even if dangerous and thrilling, can easily pall if it's just one thing after another. Foreshadowing may be needed -- it's easiest if the obstacle is one that comes somewhat after the thing that makes you realize it -- and it needs to raise the conflict, and it needs above all to fit the story's unity of theme.
On the other hand, while such events can throw unity out of whack by their irrelevancy -- there's a lion roaring about the valley, what does this have to do with the animated statuary that your sorcerer foe can make -- they do have the advantage of being about the main character. There are very few problems that are problematic in every conceivable way.