The sheep and Solas is a good example, but I was thinking about something like this:
You were able to locate the hideout of the some infamous sect/organization.
While going there you discover that nearby village is attacked by monsters/bandits/demons.
Sera wants to help villagers first and considering her personality you won't be able to change her mind (and even if you do somehow, she won't forgive herself/you for that and leave your party afterwards).
By helping villagers you'll loose the element of surprise and that might mean that the sect will notice your presence in the area and most likely flee.
By letting them escape, they will be more careful with their future operations and thus it will be harder to find their hideout again. But in the long run the damage that they will cause (land/people/Inquisition's operations) will be great.
"The" choices:
Take Sera, help the villagers, sect flees, Sera is grateful but understands consequences.
Take Sera, convince to go after sect, loose A LOT of approval.
Take Sera, go after sect, Sera defends the village but dies in the process.
Don't take Sera, go after sect, upon return to the Skyhold Sera leaves Inquisition.
Don't take Sera, help the villagers, sect flees, Sera is grateful.